FLEX(1) Cosmopolitan General Commands Manual -*-text-*- 𝐍𝐀𝐌𝐄 𝗳𝗹𝗲𝘅, 𝗳𝗹𝗲𝘅++, 𝗹𝗲𝘅 — fast lexical analyzer generator 𝐒𝐘𝐍𝐎𝐏𝐒𝐈𝐒 𝗳𝗹𝗲𝘅 [-𝟳𝟴𝐁𝗯𝗱𝐅𝗳𝗵𝐈𝗶𝐋𝗹𝗻𝗽𝘀𝐓𝘁𝐕𝘃𝘄+?] [-𝐂[𝗮𝗲𝐅𝗳𝗺𝗿]] [--𝗵𝗲𝗹𝗽] [--𝘃𝗲𝗿𝘀𝗶𝗼𝗻] [-𝗼o̲u̲t̲p̲u̲t̲] [-𝐏p̲r̲e̲f̲i̲x̲] [-𝐒s̲k̲e̲l̲e̲t̲o̲n̲] [f̲i̲l̲e̲ .̲.̲.̲] 𝐃𝐄𝐒𝐂𝐑𝐈𝐏𝐓𝐈𝐎𝐍 𝗳𝗹𝗲𝘅 is a tool for generating s̲c̲a̲n̲n̲e̲r̲s̲: programs which recognize lexical patterns in text. 𝗳𝗹𝗲𝘅 reads the given input files, or its standard input if no file names are given, for a description of a scanner to generate. The description is in the form of pairs of regular expressions and C code, called r̲u̲l̲e̲s̲. 𝗳𝗹𝗲𝘅 generates as output a C source file, l̲e̲x̲.̲y̲y̲.̲c̲, which defines a routine 𝘆𝘆𝗹𝗲𝘅(). This file is compiled and linked with the -𝗹𝗳𝗹 library to produce an executable. When the executable is run, it analyzes its input for occurrences of the regular expressions. Whenever it finds one, it executes the corresponding C code. 𝗹𝗲𝘅 is a synonym for 𝗳𝗹𝗲𝘅. 𝗳𝗹𝗲𝘅++ is a synonym for 𝗳𝗹𝗲𝘅 -+. The manual includes both tutorial and reference sections: 𝐒𝗼𝗺𝗲 𝐒𝗶𝗺𝗽𝗹𝗲 𝐄𝘅𝗮𝗺𝗽𝗹𝗲𝘀 𝐅𝗼𝗿𝗺𝗮𝘁 𝗼𝗳 𝘁𝗵𝗲 𝐈𝗻𝗽𝘂𝘁 𝐅𝗶𝗹𝗲 𝐏𝗮𝘁𝘁𝗲𝗿𝗻𝘀 The extended regular expressions used by 𝗳𝗹𝗲𝘅. 𝐇𝗼𝘄 𝘁𝗵𝗲 𝐈𝗻𝗽𝘂𝘁 𝗶𝘀 𝐌𝗮𝘁𝗰𝗵𝗲𝗱 The rules for determining what has been matched. 𝐀𝗰𝘁𝗶𝗼𝗻𝘀 How to specify what to do when a pattern is matched. 𝐓𝗵𝗲 𝐆𝗲𝗻𝗲𝗿𝗮𝘁𝗲𝗱 𝐒𝗰𝗮𝗻𝗻𝗲𝗿 Details regarding the scanner that 𝗳𝗹𝗲𝘅 produces; how to control the input source. 𝐒𝘁𝗮𝗿𝘁 𝐂𝗼𝗻𝗱𝗶𝘁𝗶𝗼𝗻𝘀 Introducing context into scanners, and managing "mini-scanners". 𝐌𝘂𝗹𝘁𝗶𝗽𝗹𝗲 𝐈𝗻𝗽𝘂𝘁 𝐁𝘂𝗳𝗳𝗲𝗿𝘀 How to manipulate multiple input sources; how to scan from strings instead of files. 𝐄𝗻𝗱-𝗼𝗳-𝐅𝗶𝗹𝗲 𝐑𝘂𝗹𝗲𝘀 Special rules for matching the end of the input. 𝐌𝗶𝘀𝗰𝗲𝗹𝗹𝗮𝗻𝗲𝗼𝘂𝘀 𝐌𝗮𝗰𝗿𝗼𝘀 A summary of macros available to the actions. 𝐕𝗮𝗹𝘂𝗲𝘀 𝐀𝘃𝗮𝗶𝗹𝗮𝗯𝗹𝗲 𝘁𝗼 𝘁𝗵𝗲 𝐔𝘀𝗲𝗿 A summary of values available to the actions. 𝐈𝗻𝘁𝗲𝗿𝗳𝗮𝗰𝗶𝗻𝗴 𝘄𝗶𝘁𝗵 𝐘𝗮𝗰𝗰 Connecting flex scanners together with yacc(1) parsers. 𝐎𝗽𝘁𝗶𝗼𝗻𝘀 𝗳𝗹𝗲𝘅 command-line options, and the “%option” directive. 𝐏𝗲𝗿𝗳𝗼𝗿𝗺𝗮𝗻𝗰𝗲 𝐂𝗼𝗻𝘀𝗶𝗱𝗲𝗿𝗮𝘁𝗶𝗼𝗻𝘀 How to make scanners go as fast as possible. 𝐆𝗲𝗻𝗲𝗿𝗮𝘁𝗶𝗻𝗴 𝐂++ 𝐒𝗰𝗮𝗻𝗻𝗲𝗿𝘀 The (experimental) facility for generating C++ scanner classes. 𝐈𝗻𝗰𝗼𝗺𝗽𝗮𝘁𝗶𝗯𝗶𝗹𝗶𝘁𝗶𝗲𝘀 𝘄𝗶𝘁𝗵 𝐋𝗲𝘅 𝗮𝗻𝗱 𝐏𝐎𝐒𝐈𝐗 How 𝗳𝗹𝗲𝘅 differs from AT&T UNIX 𝗹𝗲𝘅 and the POSIX 𝗹𝗲𝘅 standard. 𝐅𝗶𝗹𝗲𝘀 Files used by 𝗳𝗹𝗲𝘅. 𝐃𝗶𝗮𝗴𝗻𝗼𝘀𝘁𝗶𝗰𝘀 Those error messages produced by 𝗳𝗹𝗲𝘅 (or scanners it generates) whose meanings might not be apparent. 𝐒𝗲𝗲 𝐀𝗹𝘀𝗼 Other documentation, related tools. 𝐀𝘂𝘁𝗵𝗼𝗿𝘀 Includes contact information. 𝐁𝘂𝗴𝘀 Known problems with 𝗳𝗹𝗲𝘅. 𝐒𝐎𝐌𝐄 𝐒𝐈𝐌𝐏𝐋𝐄 𝐄𝐗𝐀𝐌𝐏𝐋𝐄𝐒 First some simple examples to get the flavor of how one uses 𝗳𝗹𝗲𝘅. The following 𝗳𝗹𝗲𝘅 input specifies a scanner which whenever it encounters the string "username" will replace it with the user's login name: %% username printf("%s", getlogin()); By default, any text not matched by a 𝗳𝗹𝗲𝘅 scanner is copied to the output, so the net effect of this scanner is to copy its input file to its output with each occurrence of "username" expanded. In this input, there is just one rule. "username" is the p̲a̲t̲t̲e̲r̲n̲ and the "printf" is the a̲c̲t̲i̲o̲n̲. The "%%" marks the beginning of the rules. Here's another simple example: %{ int num_lines = 0, num_chars = 0; %} %% \n ++num_lines; ++num_chars; . ++num_chars; %% main() { yylex(); printf("# of lines = %d, # of chars = %d\n", num_lines, num_chars); } This scanner counts the number of characters and the number of lines in its input (it produces no output other than the final report on the counts). The first line declares two globals, "num_lines" and "num_chars", which are accessible both inside 𝘆𝘆𝗹𝗲𝘅() and in the 𝗺𝗮𝗶𝗻() routine declared after the second "%%". There are two rules, one which matches a newline ("\n") and incre‐ ments both the line count and the character count, and one which matches any character other than a newline (indicated by the "." regular expression). A somewhat more complicated example: /* scanner for a toy Pascal-like language */ %{ /* need this for the call to atof() below */ #include %} DIGIT [0-9] ID [a-z][a-z0-9]* %% {DIGIT}+ { printf("An integer: %s (%d)\n", yytext, atoi(yytext)); } {DIGIT}+"."{DIGIT}* { printf("A float: %s (%g)\n", yytext, atof(yytext)); } if|then|begin|end|procedure|function { printf("A keyword: %s\n", yytext); } {ID} printf("An identifier: %s\n", yytext); "+"|"-"|"*"|"/" printf("An operator: %s\n", yytext); "{"[^}\n]*"}" /* eat up one-line comments */ [ \t\n]+ /* eat up whitespace */ . printf("Unrecognized character: %s\n", yytext); %% main(int argc, char *argv[]) { ++argv; --argc; /* skip over program name */ if (argc > 0) yyin = fopen(argv[0], "r"); else yyin = stdin; yylex(); } This is the beginnings of a simple scanner for a language like Pas‐ cal. It identifies different types of t̲o̲k̲e̲n̲s̲ and reports on what it has seen. The details of this example will be explained in the following sec‐ tions. 𝐅𝐎𝐑𝐌𝐀𝐓 𝐎𝐅 𝐓𝐇𝐄 𝐈𝐍𝐏𝐔𝐓 𝐅𝐈𝐋𝐄 The 𝗳𝗹𝗲𝘅 input file consists of three sections, separated by a line with just "%%" in it: definitions %% rules %% user code The d̲e̲f̲i̲n̲i̲t̲i̲o̲n̲s̲ section contains declarations of simple n̲a̲m̲e̲ defi‐ nitions to simplify the scanner specification, and declarations of s̲t̲a̲r̲t̲ c̲o̲n̲d̲i̲t̲i̲o̲n̲s̲, which are explained in a later section. Name definitions have the form: name definition The "name" is a word beginning with a letter or an underscore (‘_’) followed by zero or more letters, digits, ‘_’, or ‘-’ (dash). The definition is taken to begin at the first non-whitespace character following the name and continuing to the end of the line. The def‐ inition can subsequently be referred to using "{name}", which will expand to "(definition)". For example: DIGIT [0-9] ID [a-z][a-z0-9]* This defines "DIGIT" to be a regular expression which matches a single digit, and "ID" to be a regular expression which matches a letter followed by zero-or-more letters-or-digits. A subsequent reference to {DIGIT}+"."{DIGIT}* is identical to ([0-9])+"."([0-9])* and matches one-or-more digits followed by a ‘.’ followed by zero- or-more digits. The r̲u̲l̲e̲s̲ section of the 𝗳𝗹𝗲𝘅 input contains a series of rules of the form: pattern action The pattern must be unindented and the action must begin on the same line. See below for a further description of patterns and actions. Finally, the user code section is simply copied to l̲e̲x̲.̲y̲y̲.̲c̲ verba‐ tim. It is used for companion routines which call or are called by the scanner. The presence of this section is optional; if it is missing, the second "%%" in the input file may be skipped too. In the definitions and rules sections, any indented text or text enclosed in ‘%{’ and ‘%}’ is copied verbatim to the output (with the %{}'s removed). The %{}'s must appear unindented on lines by themselves. In the rules section, any indented or %{} text appearing before the first rule may be used to declare variables which are local to the scanning routine and (after the declarations) code which is to be executed whenever the scanning routine is entered. Other indented or %{} text in the rule section is still copied to the output, but its meaning is not well-defined and it may well cause compile-time errors (this feature is present for POSIX compliance; see below for other such features). In the definitions section (but not in the rules section), an unin‐ dented comment (i.e., a line beginning with "/*") is also copied verbatim to the output up to the next "*/". 𝐏𝐀𝐓𝐓𝐄𝐑𝐍𝐒 The patterns in the input are written using an extended set of reg‐ ular expressions. These are: x Match the character ‘x’. . Any character (byte) except newline. [xyz] A "character class"; in this case, the pattern matches either an ‘x’, a ‘y’, or a ‘z’. [abj-oZ] A "character class" with a range in it; matches an ‘a’, a ‘b’, any letter from ‘j’ through ‘o’, or a ‘Z’. [^A-Z] A "negated character class", i.e., any character but those in the class. In this case, any character EXCEPT an uppercase letter. [^A-Z\n] Any character EXCEPT an uppercase letter or a newline. r* Zero or more r's, where ‘r’ is any regular expression. r+ One or more r's. r? Zero or one r's (that is, "an optional r"). r{2,5} Anywhere from two to five r's. r{2,} Two or more r's. r{4} Exactly 4 r's. {name} The expansion of the "name" definition (see above). "[xyz]\"foo" The literal string: [xyz]"foo. \X If ‘X’ is an ‘a’, ‘b’, ‘f’, ‘n’, ‘r’, ‘t’, or ‘v’, then the ANSI-C interpretation of ‘\X’. Otherwise, a literal ‘X’ (used to escape operators such as ‘*’). \0 A NUL character (ASCII code 0). \123 The character with octal value 123. \x2a The character with hexadecimal value 2a. (r) Match an ‘r’; parentheses are used to override precedence (see below). rs The regular expression ‘r’ followed by the regular expression ‘s’; called "concatenation". r|s Either an ‘r’ or an ‘s’. r/s An ‘r’, but only if it is followed by an ‘s’. The text matched by ‘s’ is included when determining whether this rule is the "longest match", but is then returned to the input before the action is executed. So the action only sees the text matched by ‘r’. This type of pattern is called "trailing context". (There are some combinations of r/s that 𝗳𝗹𝗲𝘅 cannot match correctly; see notes in the B̲U̲G̲S̲ section below regarding "dangerous trailing context".) ^r An ‘r’, but only at the beginning of a line (i.e., just starting to scan, or right after a newline has been scanned). r$ An ‘r’, but only at the end of a line (i.e., just before a newline). Equivalent to "r/\n". Note that 𝗳𝗹𝗲𝘅's notion of "newline" is exactly whatever the C compiler used to compile 𝗳𝗹𝗲𝘅 interprets ‘\n’ as. r An ‘r’, but only in start condition ‘s’ (see below for discussion of start conditions). r The same, but in any of start conditions s1, s2, or s3. <*>r An ‘r’ in any start condition, even an exclusive one. <> An end-of-file. <> An end-of-file when in start condition s1 or s2. Note that inside of a character class, all regular expression oper‐ ators lose their special meaning except escape (‘\’) and the char‐ acter class operators, ‘-’, ‘]’, and, at the beginning of the class, ‘^’. The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom. Those grouped together have equal precedence. For exam‐ ple, foo|bar* is the same as (foo)|(ba(r*)) since the ‘*’ operator has higher precedence than concatenation, and concatenation higher than alternation (‘|’). This pattern therefore matches e̲i̲t̲h̲e̲r̲ the string "foo" o̲r̲ the string "ba" fol‐ lowed by zero-or-more r's. To match "foo" or zero-or-more "bar"'s, use: foo|(bar)* and to match zero-or-more "foo"'s-or-"bar"'s: (foo|bar)* In addition to characters and ranges of characters, character classes can also contain character class e̲x̲p̲r̲e̲s̲s̲i̲o̲n̲s̲. These are expressions enclosed inside ‘[:’ and ‘:]’ delimiters (which them‐ selves must appear between the ‘[’ and ‘]’ of the character class; other elements may occur inside the character class, too). The valid expressions are: [:alnum:] [:alpha:] [:blank:] [:cntrl:] [:digit:] [:graph:] [:lower:] [:print:] [:punct:] [:space:] [:upper:] [:xdigit:] These expressions all designate a set of characters equivalent to the corresponding standard C 𝗶𝘀𝐗𝐗𝐗() function. For example, [:alnum:] designates those characters for which isalnum(3) returns true - i.e., any alphabetic or numeric. Some systems don't provide isblank(3), so 𝗳𝗹𝗲𝘅 defines [:blank:] as a blank or a tab. For example, the following character classes are all equivalent: [[:alnum:]] [[:alpha:][:digit:]] [[:alpha:]0-9] [a-zA-Z0-9] If the scanner is case-insensitive (the -𝗶 flag), then [:upper:] and [:lower:] are equivalent to [:alpha:]. Some notes on patterns: - A negated character class such as the example "[^A-Z]" above will match a newline unless "\n" (or an equivalent escape sequence) is one of the characters explicitly present in the negated character class (e.g., "[^A-Z\n]"). This is unlike how many other regular expression tools treat negated character classes, but unfortunately the inconsistency is historically entrenched. Matching newlines means that a pattern like "[^"]*" can match the entire input unless there's another quote in the input. - A rule can have at most one instance of trailing context (the ‘/’ operator or the ‘$’ operator). The start condition, ‘^’, and "<>" patterns can only occur at the beginning of a pattern and, as well as with ‘/’ and ‘$’, cannot be grouped inside parentheses. A ‘^’ which does not occur at the begin‐ ning of a rule or a ‘$’ which does not occur at the end of a rule loses its special properties and is treated as a normal character. - The following are illegal: foo/bar$ foobar Note that the first of these, can be written "foo/bar\n". - The following will result in ‘$’ or ‘^’ being treated as a nor‐ mal character: foo|(bar$) foo|^bar If what's wanted is a "foo" or a bar-followed-by-a-newline, the following could be used (the special ‘|’ action is explained below): foo | bar$ /* action goes here */ A similar trick will work for matching a foo or a bar-at-the- beginning-of-a-line. 𝐇𝐎𝐖 𝐓𝐇𝐄 𝐈𝐍𝐏𝐔𝐓 𝐈𝐒 𝐌𝐀𝐓𝐂𝐇𝐄𝐃 When the generated scanner is run, it analyzes its input looking for strings which match any of its patterns. If it finds more than one match, it takes the one matching the most text (for trailing context rules, this includes the length of the trailing part, even though it will then be returned to the input). If it finds two or more matches of the same length, the rule listed first in the 𝗳𝗹𝗲𝘅 input file is chosen. Once the match is determined, the text corresponding to the match (called the t̲o̲k̲e̲n̲) is made available in the global character pointer y̲y̲t̲e̲x̲t̲, and its length in the global integer y̲y̲l̲e̲n̲g̲. The a̲c̲t̲i̲o̲n̲ corresponding to the matched pattern is then executed (a more detailed description of actions follows), and then the remain‐ ing input is scanned for another match. If no match is found, then the default rule is executed: the next character in the input is considered matched and copied to the standard output. Thus, the simplest legal 𝗳𝗹𝗲𝘅 input is: %% which generates a scanner that simply copies its input (one character at a time) to its output. Note that y̲y̲t̲e̲x̲t̲ can be defined in two different ways: either as a character pointer or as a character array. Which definition 𝗳𝗹𝗲𝘅 uses can be controlled by including one of the special directives “%pointer” or “%array” in the first (definitions) section of flex input. The default is “%pointer”, unless the -𝗹 𝗹𝗲𝘅 compatibility option is used, in which case y̲y̲t̲e̲x̲t̲ will be an array. The advan‐ tage of using “%pointer” is substantially faster scanning and no buffer overflow when matching very large tokens (unless not enough dynamic memory is available). The disadvantage is that actions are restricted in how they can modify y̲y̲t̲e̲x̲t̲ (see the next section), and calls to the 𝘂𝗻𝗽𝘂𝘁() function destroy the present contents of y̲y̲t̲e̲x̲t̲, which can be a considerable porting headache when moving between different 𝗹𝗲𝘅 versions. The advantage of “%array” is that y̲y̲t̲e̲x̲t̲ can be modified as much as wanted, and calls to 𝘂𝗻𝗽𝘂𝘁() do not destroy y̲y̲t̲e̲x̲t̲ (see below). Furthermore, existing 𝗹𝗲𝘅 programs sometimes access y̲y̲t̲e̲x̲t̲ exter‐ nally using declarations of the form: extern char yytext[]; This definition is erroneous when used with “%pointer”, but correct for “%array”. “%array” defines y̲y̲t̲e̲x̲t̲ to be an array of YYLMAX characters, which defaults to a fairly large value. The size can be changed by sim‐ ply #define'ing YYLMAX to a different value in the first section of 𝗳𝗹𝗲𝘅 input. As mentioned above, with “%pointer” yytext grows dynamically to accommodate large tokens. While this means a “%pointer” scanner can accommodate very large tokens (such as matching entire blocks of comments), bear in mind that each time the scanner must resize y̲y̲t̲e̲x̲t̲ it also must rescan the entire token from the beginning, so matching such tokens can prove slow. y̲y̲t̲e̲x̲t̲ presently does not dynamically grow if a call to 𝘂𝗻𝗽𝘂𝘁() results in too much text being pushed back; instead, a run-time error results. Also note that “%array” cannot be used with C++ scanner classes (the c++ option; see below). 𝐀𝐂𝐓𝐈𝐎𝐍𝐒 Each pattern in a rule has a corresponding action, which can be any arbitrary C statement. The pattern ends at the first non-escaped whitespace character; the remainder of the line is its action. If the action is empty, then when the pattern is matched the input token is simply discarded. For example, here is the specification for a program which deletes all occurrences of "zap me" from its input: %% "zap me" (It will copy all other characters in the input to the output since they will be matched by the default rule.) Here is a program which compresses multiple blanks and tabs down to a single blank, and throws away whitespace found at the end of a line: %% [ \t]+ putchar(' '); [ \t]+$ /* ignore this token */ If the action contains a ‘{’, then the action spans till the bal‐ ancing ‘}’ is found, and the action may cross multiple lines. 𝗳𝗹𝗲𝘅 knows about C strings and comments and won't be fooled by braces found within them, but also allows actions to begin with ‘%{’ and will consider the action to be all the text up to the next ‘%}’ (regardless of ordinary braces inside the action). An action consisting solely of a vertical bar (‘|’) means "same as the action for the next rule". See below for an illustration. Actions can include arbitrary C code, including return statements to return a value to whatever routine called 𝘆𝘆𝗹𝗲𝘅(). Each time 𝘆𝘆𝗹𝗲𝘅() is called, it continues processing tokens from where it last left off until it either reaches the end of the file or exe‐ cutes a return. Actions are free to modify y̲y̲t̲e̲x̲t̲ except for lengthening it (adding characters to its end - these will overwrite later characters in the input stream). This, however, does not apply when using “%array” (see above); in that case, y̲y̲t̲e̲x̲t̲ may be freely modified in any way. Actions are free to modify y̲y̲l̲e̲n̲g̲ except they should not do so if the action also includes use of 𝘆𝘆𝗺𝗼𝗿𝗲() (see below). There are a number of special directives which can be included within an action: ECHO Copies y̲y̲t̲e̲x̲t̲ to the scanner's output. BEGIN Followed by the name of a start condition, places the scan‐ ner in the corresponding start condition (see below). REJECT Directs the scanner to proceed on to the "second best" rule which matched the input (or a prefix of the input). The rule is chosen as described above in H̲O̲W̲ T̲H̲E̲ I̲N̲P̲U̲T̲ I̲S̲ M̲A̲T̲C̲H̲E̲D̲, and y̲y̲t̲e̲x̲t̲ and y̲y̲l̲e̲n̲g̲ set up appropriately. It may either be one which matched as much text as the origi‐ nally chosen rule but came later in the 𝗳𝗹𝗲𝘅 input file, or one which matched less text. For example, the following will both count the words in the input and call the routine 𝘀𝗽𝗲𝗰𝗶𝗮𝗹() whenever "frob" is seen: int word_count = 0; %% frob special(); REJECT; [^ \t\n]+ ++word_count; Without the R̲E̲J̲E̲C̲T̲, any "frob"'s in the input would not be counted as words, since the scanner normally executes only one action per token. Multiple R̲E̲J̲E̲C̲T̲'s are allowed, each one finding the next best choice to the currently active rule. For example, when the following scanner scans the token "abcd", it will write "abcdabcaba" to the output: %% a | ab | abc | abcd ECHO; REJECT; .|\n /* eat up any unmatched character */ (The first three rules share the fourth's action since they use the special ‘|’ action.) R̲E̲J̲E̲C̲T̲ is a particularly expensive feature in terms of scanner performance; if it is used in any of the scanner's actions it will slow down all of the scanner's matching. Furthermore, R̲E̲J̲E̲C̲T̲ cannot be used with the -𝐂𝗳 or -𝐂𝐅 options (see below). Note also that unlike the other special actions, R̲E̲J̲E̲C̲T̲ is a b̲r̲a̲n̲c̲h̲; code immediately following it in the action will not be executed. yymore() Tells the scanner that the next time it matches a rule, the corresponding token should be appended onto the current value of y̲y̲t̲e̲x̲t̲ rather than replacing it. For example, given the input "mega-kludge" the following will write "mega-mega-kludge" to the output: %% mega- ECHO; yymore(); kludge ECHO; First "mega-" is matched and echoed to the output. Then "kludge" is matched, but the previous "mega-" is still hanging around at the beginning of y̲y̲t̲e̲x̲t̲ so the E̲C̲H̲O̲ for the "kludge" rule will actually write "mega-kludge". Two notes regarding use of 𝘆𝘆𝗺𝗼𝗿𝗲(): First, 𝘆𝘆𝗺𝗼𝗿𝗲() depends on the value of y̲y̲l̲e̲n̲g̲ correctly reflecting the size of the current token, so y̲y̲l̲e̲n̲g̲ must not be modified when using 𝘆𝘆𝗺𝗼𝗿𝗲(). Second, the presence of 𝘆𝘆𝗺𝗼𝗿𝗲() in the scanner's action entails a minor performance penalty in the scanner's matching speed. yyless(n) Returns all but the first n̲ characters of the current token back to the input stream, where they will be rescanned when the scanner looks for the next match. y̲y̲t̲e̲x̲t̲ and y̲y̲l̲e̲n̲g̲ are adjusted appropriately (e.g., y̲y̲l̲e̲n̲g̲ will now be equal to n̲). For example, on the input "foobar" the following will write out "foobarbar": %% foobar ECHO; yyless(3); [a-z]+ ECHO; An argument of 0 to y̲y̲l̲e̲s̲s̲ will cause the entire current input string to be scanned again. Unless how the scanner will subsequently process its input has been changed (using B̲E̲G̲I̲N̲, for example), this will result in an endless loop. Note that y̲y̲l̲e̲s̲s̲ is a macro and can only be used in the 𝗳𝗹𝗲𝘅 input file, not from other source files. unput(c) Puts the character c̲ back into the input stream. It will be the next character scanned. The following action will take the current token and cause it to be rescanned enclosed in parentheses. { int i; char *yycopy; /* Copy yytext because unput() trashes yytext */ if ((yycopy = strdup(yytext)) == NULL) err(1, NULL); unput(')'); for (i = yyleng - 1; i >= 0; --i) unput(yycopy[i]); unput('('); free(yycopy); } Note that since each 𝘂𝗻𝗽𝘂𝘁() puts the given character back at the beginning of the input stream, pushing back strings must be done back-to-front. An important potential problem when using 𝘂𝗻𝗽𝘂𝘁() is that if using “%pointer” (the default), a call to 𝘂𝗻𝗽𝘂𝘁() destroys the contents of y̲y̲t̲e̲x̲t̲, starting with its right‐ most character and devouring one character to the left with each call. If the value of y̲y̲t̲e̲x̲t̲ should be preserved after a call to 𝘂𝗻𝗽𝘂𝘁() (as in the above example), it must either first be copied elsewhere, or the scanner must be built using “%array” instead (see H̲O̲W̲ T̲H̲E̲ I̲N̲P̲U̲T̲ I̲S̲ M̲A̲T̲C̲H̲E̲D̲). Finally, note that EOF cannot be put back to attempt to mark the input stream with an end-of-file. input() Reads the next character from the input stream. For exam‐ ple, the following is one way to eat up C comments: %% "/*" { int c; for (;;) { while ((c = input()) != '*' && c != EOF) ; /* eat up text of comment */ if (c == '*') { while ((c = input()) == '*') ; if (c == '/') break; /* found the end */ } if (c == EOF) { errx(1, "EOF in comment"); break; } } } (Note that if the scanner is compiled using C++, then 𝗶𝗻𝗽𝘂𝘁() is instead referred to as 𝘆𝘆𝗶𝗻𝗽𝘂𝘁(), in order to avoid a name clash with the C++ stream by the name of input.) YY_FLUSH_BUFFER Flushes the scanner's internal buffer so that the next time the scanner attempts to match a token, it will first refill the buffer using YY_INPUT (see T̲H̲E̲ G̲E̲N̲E̲R̲A̲T̲E̲D̲ S̲C̲A̲N̲N̲E̲R̲, below). This action is a special case of the more general 𝘆𝘆_𝗳𝗹𝘂𝘀𝗵_𝗯𝘂𝗳𝗳𝗲𝗿() function, described below in the section M̲U̲L̲T̲I̲P̲L̲E̲ I̲N̲P̲U̲T̲ B̲U̲F̲F̲E̲R̲S̲. yyterminate() Can be used in lieu of a return statement in an action. It terminates the scanner and returns a 0 to the scanner's caller, indicating "all done". By default, 𝘆𝘆𝘁𝗲𝗿𝗺𝗶𝗻𝗮𝘁𝗲() is also called when an end-of-file is encountered. It is a macro and may be redefined. 𝐓𝐇𝐄 𝐆𝐄𝐍𝐄𝐑𝐀𝐓𝐄𝐃 𝐒𝐂𝐀𝐍𝐍𝐄𝐑 The output of 𝗳𝗹𝗲𝘅 is the file l̲e̲x̲.̲y̲y̲.̲c̲, which contains the scan‐ ning routine 𝘆𝘆𝗹𝗲𝘅(), a number of tables used by it for matching tokens, and a number of auxiliary routines and macros. By default, 𝘆𝘆𝗹𝗲𝘅() is declared as follows: int yylex() { ... various definitions and the actions in here ... } (If the environment supports function prototypes, then it will be "int yylex(void)".) This definition may be changed by defining the YY_DECL macro. For example: #define YY_DECL float lexscan(a, b) float a, b; would give the scanning routine the name l̲e̲x̲s̲c̲a̲n̲, returning a float, and taking two floats as arguments. Note that if arguments are given to the scanning routine using a K&R-style/non-prototyped function declaration, the definition must be terminated with a semi-colon (‘;’). Whenever 𝘆𝘆𝗹𝗲𝘅() is called, it scans tokens from the global input file y̲y̲i̲n̲ (which defaults to stdin). It continues until it either reaches an end-of-file (at which point it returns the value 0) or one of its actions executes a r̲e̲t̲u̲r̲n̲ statement. If the scanner reaches an end-of-file, subsequent calls are unde‐ fined unless either y̲y̲i̲n̲ is pointed at a new input file (in which case scanning continues from that file), or 𝘆𝘆𝗿𝗲𝘀𝘁𝗮𝗿𝘁() is called. 𝘆𝘆𝗿𝗲𝘀𝘁𝗮𝗿𝘁() takes one argument, a F̲I̲L̲E̲ *̲ pointer (which can be nil, if YY_INPUT has been set up to scan from a source other than y̲y̲i̲n̲), and initializes y̲y̲i̲n̲ for scanning from that file. Essentially there is no difference between just assigning y̲y̲i̲n̲ to a new input file or using 𝘆𝘆𝗿𝗲𝘀𝘁𝗮𝗿𝘁() to do so; the latter is available for compatibility with previous versions of 𝗳𝗹𝗲𝘅, and because it can be used to switch input files in the middle of scanning. It can also be used to throw away the current input buffer, by calling it with an argument of y̲y̲i̲n̲; but better is to use YY_FLUSH_BUFFER (see above). Note that 𝘆𝘆𝗿𝗲𝘀𝘁𝗮𝗿𝘁() does not reset the start condition to I̲N̲I̲T̲I̲A̲L̲ (see S̲T̲A̲R̲T̲ C̲O̲N̲D̲I̲T̲I̲O̲N̲S̲, below). If 𝘆𝘆𝗹𝗲𝘅() stops scanning due to executing a r̲e̲t̲u̲r̲n̲ statement in one of the actions, the scanner may then be called again and it will resume scanning where it left off. By default (and for purposes of efficiency), the scanner uses block-reads rather than simple getc(3) calls to read characters from y̲y̲i̲n̲. The nature of how it gets its input can be controlled by defining the YY_INPUT macro. YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)". Its action is to place up to max_size characters in the character array b̲u̲f̲ and return in the integer variable r̲e̲s̲u̲l̲t̲ either the number of characters read or the constant YY_NULL (0 on UNIX systems) to indicate EOF. The default YY_INPUT reads from the global file-pointer "yyin". A sample definition of YY_INPUT (in the definitions section of the input file): %{ #define YY_INPUT(buf,result,max_size) \ { \ int c = getchar(); \ result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \ } %} This definition will change the input processing to occur one char‐ acter at a time. When the scanner receives an end-of-file indication from YY_INPUT, it then checks the 𝘆𝘆𝘄𝗿𝗮𝗽() function. If 𝘆𝘆𝘄𝗿𝗮𝗽() returns false (zero), then it is assumed that the function has gone ahead and set up y̲y̲i̲n̲ to point to another input file, and scanning continues. If it returns true (non-zero), then the scanner terminates, returning 0 to its caller. Note that in either case, the start condition remains unchanged; it does not revert to I̲N̲I̲T̲I̲A̲L̲. If you do not supply your own version of 𝘆𝘆𝘄𝗿𝗮𝗽(), then you must either use “%option noyywrap” (in which case the scanner behaves as though 𝘆𝘆𝘄𝗿𝗮𝗽() returned 1), or you must link with -𝗹𝗳𝗹 to obtain the default version of the routine, which always returns 1. Three routines are available for scanning from in-memory buffers rather than files: 𝘆𝘆_𝘀𝗰𝗮𝗻_𝘀𝘁𝗿𝗶𝗻𝗴(), 𝘆𝘆_𝘀𝗰𝗮𝗻_𝗯𝘆𝘁𝗲𝘀(), and 𝘆𝘆_𝘀𝗰𝗮𝗻_𝗯𝘂𝗳𝗳𝗲𝗿(). See the discussion of them below in the section M̲U̲L̲T̲I̲P̲L̲E̲ I̲N̲P̲U̲T̲ B̲U̲F̲F̲E̲R̲S̲. The scanner writes its E̲C̲H̲O̲ output to the y̲y̲o̲u̲t̲ global (default, stdout), which may be redefined by the user simply by assigning it to some other F̲I̲L̲E̲ pointer. 𝐒𝐓𝐀𝐑𝐓 𝐂𝐎𝐍𝐃𝐈𝐓𝐈𝐎𝐍𝐒 𝗳𝗹𝗲𝘅 provides a mechanism for conditionally activating rules. Any rule whose pattern is prefixed with "⟨sc⟩" will only be active when the scanner is in the start condition named "sc". For example, [^"]* { /* eat up the string body ... */ ... } will be active only when the scanner is in the "STRING" start con‐ dition, and \. { /* handle an escape ... */ ... } will be active only when the current start condition is either "INITIAL", "STRING", or "QUOTE". Start conditions are declared in the definitions (first) section of the input using unindented lines beginning with either ‘%s’ or ‘%x’ followed by a list of names. The former declares i̲n̲c̲l̲u̲s̲i̲v̲e̲ start conditions, the latter e̲x̲c̲l̲u̲s̲i̲v̲e̲ start conditions. A start condi‐ tion is activated using the B̲E̲G̲I̲N̲ action. Until the next B̲E̲G̲I̲N̲ action is executed, rules with the given start condition will be active and rules with other start conditions will be inactive. If the start condition is inclusive, then rules with no start condi‐ tions at all will also be active. If it is exclusive, then only rules qualified with the start condition will be active. A set of rules contingent on the same exclusive start condition describe a scanner which is independent of any of the other rules in the 𝗳𝗹𝗲𝘅 input. Because of this, exclusive start conditions make it easy to specify "mini-scanners" which scan portions of the input that are syntactically different from the rest (e.g., comments). If the distinction between inclusive and exclusive start conditions is still a little vague, here's a simple example illustrating the connection between the two. The set of rules: %s example %% foo do_something(); bar something_else(); is equivalent to %x example %% foo do_something(); bar something_else(); Without the ⟨INITIAL,example⟩ qualifier, the “bar” pattern in the second example wouldn't be active (i.e., couldn't match) when in start condition “example”. If we just used ⟨example⟩ to qualify “bar”, though, then it would only be active in “example” and not in I̲N̲I̲T̲I̲A̲L̲, while in the first example it's active in both, because in the first example the “example” start condition is an inclusive (‘%s’) start condition. Also note that the special start-condition specifier ‘⟨*⟩’ matches every start condition. Thus, the above example could also have been written: %x example %% foo do_something(); <*>bar something_else(); The default rule (to E̲C̲H̲O̲ any unmatched character) remains active in start conditions. It is equivalent to: <*>.|\n ECHO; “BEGIN(0)” returns to the original state where only the rules with no start conditions are active. This state can also be referred to as the start-condition I̲N̲I̲T̲I̲A̲L̲, so “BEGIN(INITIAL)” is equivalent to “BEGIN(0)”. (The parentheses around the start condition name are not required but are considered good style.) B̲E̲G̲I̲N̲ actions can also be given as indented code at the beginning of the rules section. For example, the following will cause the scanner to enter the "SPECIAL" start condition whenever 𝘆𝘆𝗹𝗲𝘅() is called and the global variable e̲n̲t̲e̲r̲_s̲p̲e̲c̲i̲a̲l̲ is true: int enter_special; %x SPECIAL %% if (enter_special) BEGIN(SPECIAL); blahblahblah ...more rules follow... To illustrate the uses of start conditions, here is a scanner which provides two different interpretations of a string like "123.456". By default it will treat it as three tokens: the integer "123", a dot (‘.’), and the integer "456". But if the string is preceded earlier in the line by the string "expect-floats" it will treat it as a single token, the floating-point number 123.456: %{ #include %} %s expect %% expect-floats BEGIN(expect); [0-9]+"."[0-9]+ { printf("found a float, = %f\n", atof(yytext)); } \n { /* * That's the end of the line, so * we need another "expect-number" * before we'll recognize any more * numbers. */ BEGIN(INITIAL); } [0-9]+ { printf("found an integer, = %d\n", atoi(yytext)); } "." printf("found a dot\n"); Here is a scanner which recognizes (and discards) C comments while maintaining a count of the current input line: %x comment %% int line_num = 1; "/*" BEGIN(comment); [^*\n]* /* eat anything that's not a '*' */ "*"+[^*/\n]* /* eat up '*'s not followed by '/'s */ \n ++line_num; "*"+"/" BEGIN(INITIAL); This scanner goes to a bit of trouble to match as much text as pos‐ sible with each rule. In general, when attempting to write a high- speed scanner try to match as much as possible in each rule, as it's a big win. Note that start-condition names are really integer values and can be stored as such. Thus, the above could be extended in the fol‐ lowing fashion: %x comment foo %% int line_num = 1; int comment_caller; "/*" { comment_caller = INITIAL; BEGIN(comment); } ... "/*" { comment_caller = foo; BEGIN(comment); } [^*\n]* /* eat anything that's not a '*' */ "*"+[^*/\n]* /* eat up '*'s not followed by '/'s */ \n ++line_num; "*"+"/" BEGIN(comment_caller); Furthermore, the current start condition can be accessed by using the integer-valued YY_START macro. For example, the above assign‐ ments to c̲o̲m̲m̲e̲n̲t̲_c̲a̲l̲l̲e̲r̲ could instead be written comment_caller = YY_START; Flex provides YYSTATE as an alias for YY_START (since that is what's used by AT&T UNIX 𝗹𝗲𝘅). Note that start conditions do not have their own name-space; %s's and %x's declare names in the same fashion as #define's. Finally, here's an example of how to match C-style quoted strings using exclusive start conditions, including expanded escape sequences (but not including checking for a string that's too long): %x str %% #define MAX_STR_CONST 1024 char string_buf[MAX_STR_CONST]; char *string_buf_ptr; \" string_buf_ptr = string_buf; BEGIN(str); \" { /* saw closing quote - all done */ BEGIN(INITIAL); *string_buf_ptr = '\0'; /* * return string constant token type and * value to parser */ } \n { /* error - unterminated string constant */ /* generate error message */ } \\[0-7]{1,3} { /* octal escape sequence */ int result; (void) sscanf(yytext + 1, "%o", &result); if (result > 0xff) { /* error, constant is out-of-bounds */ } else *string_buf_ptr++ = result; } \\[0-9]+ { /* * generate error - bad escape sequence; something * like '\48' or '\0777777' */ } \\n *string_buf_ptr++ = '\n'; \\t *string_buf_ptr++ = '\t'; \\r *string_buf_ptr++ = '\r'; \\b *string_buf_ptr++ = '\b'; \\f *string_buf_ptr++ = '\f'; \\(.|\n) *string_buf_ptr++ = yytext[1]; [^\\\n\"]+ { char *yptr = yytext; while (*yptr) *string_buf_ptr++ = *yptr++; } Often, such as in some of the examples above, a whole bunch of rules are all preceded by the same start condition(s). 𝗳𝗹𝗲𝘅 makes this a little easier and cleaner by introducing a notion of start condition s̲c̲o̲p̲e̲. A start condition scope is begun with: { where “SCs” is a list of one or more start conditions. Inside the start condition scope, every rule automatically has the prefix ⟨SCs⟩ applied to it, until a ‘}’ which matches the initial ‘{’. So, for example, { "\\n" return '\n'; "\\r" return '\r'; "\\f" return '\f'; "\\0" return '\0'; } is equivalent to: "\\n" return '\n'; "\\r" return '\r'; "\\f" return '\f'; "\\0" return '\0'; Start condition scopes may be nested. Three routines are available for manipulating stacks of start con‐ ditions: void yy_push_state(int new_state) Pushes the current start condition onto the top of the start condition stack and switches to n̲e̲w̲_s̲t̲a̲t̲e̲ as though “BEGIN new_state” had been used (recall that start condition names are also integers). void yy_pop_state() Pops the top of the stack and switches to it via B̲E̲G̲I̲N̲. int yy_top_state() Returns the top of the stack without altering the stack's contents. The start condition stack grows dynamically and so has no built-in size limitation. If memory is exhausted, program execution aborts. To use start condition stacks, scanners must include a “%option stack” directive (see O̲P̲T̲I̲O̲N̲S̲ below). 𝐌𝐔𝐋𝐓𝐈𝐏𝐋𝐄 𝐈𝐍𝐏𝐔𝐓 𝐁𝐔𝐅𝐅𝐄𝐑𝐒 Some scanners (such as those which support "include" files) require reading from several input streams. As 𝗳𝗹𝗲𝘅 scanners do a large amount of buffering, one cannot control where the next input will be read from by simply writing a YY_INPUT which is sensitive to the scanning context. YY_INPUT is only called when the scanner reaches the end of its buffer, which may be a long time after scanning a statement such as an "include" which requires switching the input source. To negotiate these sorts of problems, 𝗳𝗹𝗲𝘅 provides a mechanism for creating and switching between multiple input buffers. An input buffer is created by using: YY_BUFFER_STATE yy_create_buffer(FILE *file, int size) which takes a F̲I̲L̲E̲ pointer and a s̲i̲z̲e̲ and creates a buffer associ‐ ated with the given file and large enough to hold s̲i̲z̲e̲ characters (when in doubt, use YY_BUF_SIZE for the size). It returns a YY_BUFFER_STATE handle, which may then be passed to other routines (see below). The YY_BUFFER_STATE type is a pointer to an opaque “struct yy_buffer_state” structure, so YY_BUFFER_STATE variables may be safely initialized to “((YY_BUFFER_STATE) 0)” if desired, and the opaque structure can also be referred to in order to cor‐ rectly declare input buffers in source files other than that of scanners. Note that the F̲I̲L̲E̲ pointer in the call to 𝘆𝘆_𝗰𝗿𝗲𝗮𝘁𝗲_𝗯𝘂𝗳𝗳𝗲𝗿() is only used as the value of y̲y̲i̲n̲ seen by YY_INPUT; if YY_INPUT is redefined so that it no longer uses y̲y̲i̲n̲, then a nil F̲I̲L̲E̲ pointer can safely be passed to 𝘆𝘆_𝗰𝗿𝗲𝗮𝘁𝗲_𝗯𝘂𝗳𝗳𝗲𝗿(). To select a particular buffer to scan: void yy_switch_to_buffer(YY_BUFFER_STATE new_buffer) It switches the scanner's input buffer so subsequent tokens will come from n̲e̲w̲_b̲u̲f̲f̲e̲r̲. Note that 𝘆𝘆_𝘀𝘄𝗶𝘁𝗰𝗵_𝘁𝗼_𝗯𝘂𝗳𝗳𝗲𝗿() may be used by 𝘆𝘆𝘄𝗿𝗮𝗽() to set things up for continued scanning, instead of opening a new file and pointing y̲y̲i̲n̲ at it. Note also that switch‐ ing input sources via either 𝘆𝘆_𝘀𝘄𝗶𝘁𝗰𝗵_𝘁𝗼_𝗯𝘂𝗳𝗳𝗲𝗿() or 𝘆𝘆𝘄𝗿𝗮𝗽() does not change the start condition. void yy_delete_buffer(YY_BUFFER_STATE buffer) is used to reclaim the storage associated with a buffer. (b̲u̲f̲f̲e̲r̲ can be nil, in which case the routine does nothing.) To clear the current contents of a buffer: void yy_flush_buffer(YY_BUFFER_STATE buffer) This function discards the buffer's contents, so the next time the scanner attempts to match a token from the buffer, it will first fill the buffer anew using YY_INPUT. 𝘆𝘆_𝗻𝗲𝘄_𝗯𝘂𝗳𝗳𝗲𝗿() is an alias for 𝘆𝘆_𝗰𝗿𝗲𝗮𝘁𝗲_𝗯𝘂𝗳𝗳𝗲𝗿(), provided for compatibility with the C++ use of n̲e̲w̲ and d̲e̲l̲e̲t̲e̲ for creating and destroying dynamic objects. Finally, the YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE han‐ dle to the current buffer. Here is an example of using these features for writing a scanner which expands include files (the ⟨⟨EOF⟩⟩ feature is discussed below): /* * the "incl" state is used for picking up the name * of an include file */ %x incl %{ #define MAX_INCLUDE_DEPTH 10 YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH]; int include_stack_ptr = 0; %} %% include BEGIN(incl); [a-z]+ ECHO; [^a-z\n]*\n? ECHO; [ \t]* /* eat the whitespace */ [^ \t\n]+ { /* got the include file name */ if (include_stack_ptr >= MAX_INCLUDE_DEPTH) errx(1, "Includes nested too deeply"); include_stack[include_stack_ptr++] = YY_CURRENT_BUFFER; yyin = fopen(yytext, "r"); if (yyin == NULL) err(1, NULL); yy_switch_to_buffer( yy_create_buffer(yyin, YY_BUF_SIZE)); BEGIN(INITIAL); } <> { if (--include_stack_ptr < 0) yyterminate(); else { yy_delete_buffer(YY_CURRENT_BUFFER); yy_switch_to_buffer( include_stack[include_stack_ptr]); } } Three routines are available for setting up input buffers for scan‐ ning in-memory strings instead of files. All of them create a new input buffer for scanning the string, and return a corresponding YY_BUFFER_STATE handle (which should be deleted afterwards using 𝘆𝘆_𝗱𝗲𝗹𝗲𝘁𝗲_𝗯𝘂𝗳𝗳𝗲𝗿()). They also switch to the new buffer using 𝘆𝘆_𝘀𝘄𝗶𝘁𝗰𝗵_𝘁𝗼_𝗯𝘂𝗳𝗳𝗲𝗿(), so the next call to 𝘆𝘆𝗹𝗲𝘅() will start scan‐ ning the string. yy_scan_string(const char *str) Scans a NUL-terminated string. yy_scan_bytes(const char *bytes, int len) Scans l̲e̲n̲ bytes (including possibly NUL's) starting at location b̲y̲t̲e̲s̲. Note that both of these functions create and scan a copy of the string or bytes. (This may be desirable, since 𝘆𝘆𝗹𝗲𝘅() modifies the contents of the buffer it is scanning.) The copy can be avoided by using: yy_scan_buffer(char *base, yy_size_t size) Which scans the buffer starting at b̲a̲s̲e̲, consisting of s̲i̲z̲e̲ bytes, the last two bytes of which must be YY_END_OF_BUFFER_CHAR (ASCII NUL). These last two bytes are not scanned; thus, scanning consists of base[0] through base[size-2], inclusive. If b̲a̲s̲e̲ is not set up in this manner (i.e., forget the final two YY_END_OF_BUFFER_CHAR bytes), then 𝘆𝘆_𝘀𝗰𝗮𝗻_𝗯𝘂𝗳𝗳𝗲𝗿() returns a nil pointer instead of creating a new input buffer. The type y̲y̲_s̲i̲z̲e̲_t̲ is an integral type which can be cast to an integer expression reflecting the size of the buffer. 𝐄𝐍𝐃-𝐎𝐅-𝐅𝐈𝐋𝐄 𝐑𝐔𝐋𝐄𝐒 The special rule "⟨⟨EOF⟩⟩" indicates actions which are to be taken when an end-of-file is encountered and 𝘆𝘆𝘄𝗿𝗮𝗽() returns non-zero (i.e., indicates no further files to process). The action must finish by doing one of four things: - Assigning y̲y̲i̲n̲ to a new input file (in previous versions of 𝗳𝗹𝗲𝘅, after doing the assignment, it was necessary to call the special action YY_NEW_FILE; this is no longer necessary). - Executing a r̲e̲t̲u̲r̲n̲ statement. - Executing the special 𝘆𝘆𝘁𝗲𝗿𝗺𝗶𝗻𝗮𝘁𝗲() action. - Switching to a new buffer using 𝘆𝘆_𝘀𝘄𝗶𝘁𝗰𝗵_𝘁𝗼_𝗯𝘂𝗳𝗳𝗲𝗿() as shown in the example above. ⟨⟨EOF⟩⟩ rules may not be used with other patterns; they may only be qualified with a list of start conditions. If an unqualified ⟨⟨EOF⟩⟩ rule is given, it applies to all start conditions which do not already have ⟨⟨EOF⟩⟩ actions. To specify an ⟨⟨EOF⟩⟩ rule for only the initial start condition, use <> These rules are useful for catching things like unclosed comments. An example: %x quote %% ...other rules for dealing with quotes... <> { error("unterminated quote"); yyterminate(); } <> { if (*++filelist) yyin = fopen(*filelist, "r"); else yyterminate(); } 𝐌𝐈𝐒𝐂𝐄𝐋𝐋𝐀𝐍𝐄𝐎𝐔𝐒 𝐌𝐀𝐂𝐑𝐎𝐒 The macro YY_USER_ACTION can be defined to provide an action which is always executed prior to the matched rule's action. For exam‐ ple, it could be #define'd to call a routine to convert yytext to lower-case. When YY_USER_ACTION is invoked, the variable y̲y̲_a̲c̲t̲ gives the number of the matched rule (rules are numbered starting with 1). For example, to profile how often each rule is matched, the following would do the trick: #define YY_USER_ACTION ++ctr[yy_act] where c̲t̲r̲ is an array to hold the counts for the different rules. Note that the macro YY_NUM_RULES gives the total number of rules (including the default rule, even if -𝘀 is used), so a correct dec‐ laration for c̲t̲r̲ is: int ctr[YY_NUM_RULES]; The macro YY_USER_INIT may be defined to provide an action which is always executed before the first scan (and before the scanner's internal initializations are done). For example, it could be used to call a routine to read in a data table or open a logging file. The macro yy_set_interactive(is_interactive) can be used to control whether the current buffer is considered i̲n̲t̲e̲r̲a̲c̲t̲i̲v̲e̲. An interac‐ tive buffer is processed more slowly, but must be used when the scanner's input source is indeed interactive to avoid problems due to waiting to fill buffers (see the discussion of the -𝐈 flag below). A non-zero value in the macro invocation marks the buffer as interactive, a zero value as non-interactive. Note that use of this macro overrides “%option always-interactive” or “%option never-interactive” (see O̲P̲T̲I̲O̲N̲S̲ below). 𝘆𝘆_𝘀𝗲𝘁_𝗶𝗻𝘁𝗲𝗿𝗮𝗰𝘁𝗶𝘃𝗲() must be invoked prior to beginning to scan the buffer that is (or is not) to be considered interactive. The macro yy_set_bol(at_bol) can be used to control whether the current buffer's scanning context for the next token match is done as though at the beginning of a line. A non-zero macro argument makes rules anchored with ‘^’ active, while a zero argument makes ‘^’ rules inactive. The macro YY_AT_BOL returns true if the next token scanned from the current buffer will have ‘^’ rules active, false otherwise. In the generated scanner, the actions are all gathered in one large switch statement and separated using YY_BREAK, which may be rede‐ fined. By default, it is simply a "break", to separate each rule's action from the following rules. Redefining YY_BREAK allows, for example, C++ users to “#define YY_BREAK” to do nothing (while being very careful that every rule ends with a "break" or a "return"!) to avoid suffering from unreachable statement warnings where because a rule's action ends with “return”, the YY_BREAK is inac‐ cessible. 𝐕𝐀𝐋𝐔𝐄𝐒 𝐀𝐕𝐀𝐈𝐋𝐀𝐁𝐋𝐄 𝐓𝐎 𝐓𝐇𝐄 𝐔𝐒𝐄𝐑 This section summarizes the various values available to the user in the rule actions. char *yytext Holds the text of the current token. It may be modified but not lengthened (characters cannot be appended to the end). If the special directive “%array” appears in the first sec‐ tion of the scanner description, then y̲y̲t̲e̲x̲t̲ is instead declared “char yytext[YYLMAX]”, where YYLMAX is a macro definition that can be redefined in the first section to change the default value (generally 8KB). Using “%array” results in somewhat slower scanners, but the value of y̲y̲t̲e̲x̲t̲ becomes immune to calls to 𝗶𝗻𝗽𝘂𝘁() and 𝘂𝗻𝗽𝘂𝘁(), which potentially destroy its value when y̲y̲t̲e̲x̲t̲ is a char‐ acter pointer. The opposite of “%array” is “%pointer”, which is the default. “%array” cannot be used when generating C++ scanner classes (the -+ flag). int yyleng Holds the length of the current token. FILE *yyin Is the file which by default 𝗳𝗹𝗲𝘅 reads from. It may be redefined, but doing so only makes sense before scanning begins or after an EOF has been encountered. Changing it in the midst of scanning will have unexpected results since 𝗳𝗹𝗲𝘅 buffers its input; use 𝘆𝘆𝗿𝗲𝘀𝘁𝗮𝗿𝘁() instead. Once scanning terminates because an end-of-file has been seen, y̲y̲i̲n̲ can be assigned as the new input file and the scanner can be called again to continue scanning. void yyrestart(FILE *new_file) May be called to point y̲y̲i̲n̲ at the new input file. The switch-over to the new file is immediate (any previously buffered-up input is lost). Note that calling 𝘆𝘆𝗿𝗲𝘀𝘁𝗮𝗿𝘁() with y̲y̲i̲n̲ as an argument thus throws away the current input buffer and continues scanning the same input file. FILE *yyout Is the file to which E̲C̲H̲O̲ actions are done. It can be reassigned by the user. YY_CURRENT_BUFFER Returns a YY_BUFFER_STATE handle to the current buffer. YY_START Returns an integer value corresponding to the current start condition. This value can subsequently be used with B̲E̲G̲I̲N̲ to return to that start condition. 𝐈𝐍𝐓𝐄𝐑𝐅𝐀𝐂𝐈𝐍𝐆 𝐖𝐈𝐓𝐇 𝐘𝐀𝐂𝐂 One of the main uses of 𝗳𝗹𝗲𝘅 is as a companion to the yacc(1) parser-generator. yacc parsers expect to call a routine named 𝘆𝘆𝗹𝗲𝘅() to find the next input token. The routine is supposed to return the type of the next token as well as putting any associated value in the global y̲y̲l̲v̲a̲l̲, which is defined externally, and can be a union or any other complex data structure. To use 𝗳𝗹𝗲𝘅 with yacc, one specifies the -𝗱 option to yacc to instruct it to gener‐ ate the file y̲.̲t̲a̲b̲.̲h̲ containing definitions of all the “%tokens” appearing in the yacc input. This file is then included in the 𝗳𝗹𝗲𝘅 scanner. For example, if one of the tokens is "TOK_NUMBER", part of the scanner might look like: %{ #include "y.tab.h" %} %% [0-9]+ yylval = atoi(yytext); return TOK_NUMBER; 𝐎𝐏𝐓𝐈𝐎𝐍𝐒 𝗳𝗹𝗲𝘅 has the following options: -𝟳 Instructs 𝗳𝗹𝗲𝘅 to generate a 7-bit scanner, i.e., one which can only recognize 7-bit characters in its input. The advantage of using -𝟳 is that the scanner's tables can be up to half the size of those generated using the -𝟴 option (see below). The disadvantage is that such scanners often hang or crash if their input contains an 8-bit character. Note, however, that unless generating a scanner using the -𝐂𝗳 or -𝐂𝐅 table compression options, use of -𝟳 will save only a small amount of table space, and make the scanner considerably less portable. 𝗳𝗹𝗲𝘅's default behavior is to generate an 8-bit scanner unless -𝐂𝗳 or -𝐂𝐅 is specified, in which case 𝗳𝗹𝗲𝘅 defaults to generating 7-bit scanners unless it was configured to generate 8-bit scanners (as will often be the case with non-USA sites). It is possible tell whether 𝗳𝗹𝗲𝘅 generated a 7-bit or an 8-bit scanner by inspecting the flag summary in the -𝘃 output as described below. Note that if -𝐂𝗳𝗲 or -𝐂𝐅𝗲 are used (the table compression options, but also using equivalence classes as discussed below), 𝗳𝗹𝗲𝘅 still defaults to generating an 8-bit scanner, since usually with these compression options full 8-bit tables are not much more expensive than 7-bit tables. -𝟴 Instructs 𝗳𝗹𝗲𝘅 to generate an 8-bit scanner, i.e., one which can recognize 8-bit characters. This flag is only needed for scanners generated using -𝐂𝗳 or -𝐂𝐅, as other‐ wise 𝗳𝗹𝗲𝘅 defaults to generating an 8-bit scanner anyway. See the discussion of -𝟳 above for 𝗳𝗹𝗲𝘅's default behavior and the tradeoffs between 7-bit and 8-bit scanners. -𝐁 Instructs 𝗳𝗹𝗲𝘅 to generate a b̲a̲t̲c̲h̲ scanner, the opposite of i̲n̲t̲e̲r̲a̲c̲t̲i̲v̲e̲ scanners generated by -𝐈 (see below). In gen‐ eral, -𝐁 is used when the scanner will never be used inter‐ actively, and you want to squeeze a little more performance out of it. If the aim is instead to squeeze out a lot more performance, use the -𝐂𝗳 or -𝐂𝐅 options (discussed below), which turn on -𝐁 automatically anyway. -𝗯 Generate backing-up information to l̲e̲x̲.̲b̲a̲c̲k̲u̲p̲. This is a list of scanner states which require backing up and the input characters on which they do so. By adding rules one can remove backing-up states. If all backing-up states are eliminated and -𝐂𝗳 or -𝐂𝐅 is used, the generated scanner will run faster (see the -𝗽 flag). Only users who wish to squeeze every last cycle out of their scanners need worry about this option. (See the section on P̲E̲R̲F̲O̲R̲M̲A̲N̲C̲E̲ C̲O̲N̲S̲I̲D̲E̲R̲A̲T̲I̲O̲N̲S̲ below.) -𝐂[𝗮𝗲𝐅𝗳𝗺𝗿] Controls the degree of table compression and, more gener‐ ally, trade-offs between small scanners and fast scanners. -𝐂𝗮 Instructs 𝗳𝗹𝗲𝘅 to trade off larger tables in the generated scanner for faster performance because the elements of the tables are better aligned for memory access and computation. On some RISC archi‐ tectures, fetching and manipulating longwords is more efficient than with smaller-sized units such as shortwords. This option can double the size of the tables used by the scanner. -𝐂𝗲 Directs 𝗳𝗹𝗲𝘅 to construct e̲q̲u̲i̲v̲a̲l̲e̲n̲c̲e̲ c̲l̲a̲s̲s̲e̲s̲, i.e., sets of characters which have identical lexi‐ cal properties (for example, if the only appearance of digits in the 𝗳𝗹𝗲𝘅 input is in the character class "[0-9]" then the digits ‘0’, ‘1’, ‘...’, ‘9’ will all be put in the same equivalence class). Equivalence classes usually give dramatic reduc‐ tions in the final table/object file sizes (typically a factor of 2-5) and are pretty cheap performance-wise (one array look-up per character scanned). -𝐂𝐅 Specifies that the alternate fast scanner represen‐ tation (described below under the -𝐅 option) should be used. This option cannot be used with -+. -𝐂𝗳 Specifies that the f̲u̲l̲l̲ scanner tables should be generated - 𝗳𝗹𝗲𝘅 should not compress the tables by taking advantage of similar transition functions for different states. -𝐂𝗺 Directs 𝗳𝗹𝗲𝘅 to construct m̲e̲t̲a̲-̲e̲q̲u̲i̲v̲a̲l̲e̲n̲c̲e̲ c̲l̲a̲s̲s̲e̲s̲, which are sets of equivalence classes (or charac‐ ters, if equivalence classes are not being used) that are commonly used together. Meta-equivalence classes are often a big win when using compressed tables, but they have a moderate performance impact (one or two "if" tests and one array look-up per character scanned). -𝐂𝗿 Causes the generated scanner to b̲y̲p̲a̲s̲s̲ use of the standard I/O library (stdio) for input. Instead of calling fread(3) or getc(3), the scanner will use the read(2) system call, resulting in a performance gain which varies from system to system, but in general is probably negligible unless -𝐂𝗳 or -𝐂𝐅 are being used. Using -𝐂𝗿 can cause strange behav‐ ior if, for example, reading from y̲y̲i̲n̲ using stdio prior to calling the scanner (because the scanner will miss whatever text previous reads left in the stdio input buffer). -𝐂𝗿 has no effect if YY_INPUT is defined (see T̲H̲E̲ G̲E̲N̲E̲R̲A̲T̲E̲D̲ S̲C̲A̲N̲N̲E̲R̲ above). A lone -𝐂 specifies that the scanner tables should be com‐ pressed but neither equivalence classes nor meta-equiva‐ lence classes should be used. The options -𝐂𝗳 or -𝐂𝐅 and -𝐂𝗺 do not make sense together - there is no opportunity for meta-equivalence classes if the table is not being compressed. Otherwise the options may be freely mixed, and are cumulative. The default setting is -𝐂𝗲𝗺 which specifies that 𝗳𝗹𝗲𝘅 should generate equivalence classes and meta-equivalence classes. This setting provides the highest degree of table compression. It is possible to trade off faster-executing scanners at the cost of larger tables with the following generally being true: slowest & smallest -Cem -Cm -Ce -C -C{f,F}e -C{f,F} -C{f,F}a fastest & largest Note that scanners with the smallest tables are usually generated and compiled the quickest, so during development the default is usually best, maximal compression. -𝐂𝗳𝗲 is often a good compromise between speed and size for production scanners. -𝗱 Makes the generated scanner run in debug mode. Whenever a pattern is recognized and the global y̲y̲_f̲l̲e̲x̲_d̲e̲b̲u̲g̲ is non- zero (which is the default), the scanner will write to stderr a line of the form: --accepting rule at line 53 ("the matched text") The line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to 𝗳𝗹𝗲𝘅). Messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; at this point, the two look the same as far as the scanner's concerned), or reaches an end-of-file. -𝐅 Specifies that the fast scanner table representation should be used (and stdio bypassed). This representation is about as fast as the full table representation (-𝗳), and for some sets of patterns will be considerably smaller (and for others, larger). In general, if the pattern set contains both "keywords" and a catch-all, "identifier" rule, such as in the set: "case" return TOK_CASE; "switch" return TOK_SWITCH; ... "default" return TOK_DEFAULT; [a-z]+ return TOK_ID; then it's better to use the full table representation. If only the "identifier" rule is present and a hash table or some such is used to detect the keywords, it's better to use -𝐅. This option is equivalent to -𝐂𝐅𝗿 (see above). It cannot be used with -+. -𝗳 Specifies f̲a̲s̲t̲ s̲c̲a̲n̲n̲e̲r̲. No table compression is done and stdio is bypassed. The result is large but fast. This option is equivalent to -𝐂𝗳𝗿 (see above). -𝗵 Generates a help summary of 𝗳𝗹𝗲𝘅's options to stdout and then exits. -? and --𝗵𝗲𝗹𝗽 are synonyms for -𝗵. -𝐈 Instructs 𝗳𝗹𝗲𝘅 to generate an i̲n̲t̲e̲r̲a̲c̲t̲i̲v̲e̲ scanner. An interactive scanner is one that only looks ahead to decide what token has been matched if it absolutely must. It turns out that always looking one extra character ahead, even if the scanner has already seen enough text to disam‐ biguate the current token, is a bit faster than only look‐ ing ahead when necessary. But scanners that always look ahead give dreadful interactive performance; for example, when a user types a newline, it is not recognized as a new‐ line token until they enter a̲n̲o̲t̲h̲e̲r̲ token, which often means typing in another whole line. 𝗳𝗹𝗲𝘅 scanners default to i̲n̲t̲e̲r̲a̲c̲t̲i̲v̲e̲ unless -𝐂𝗳 or -𝐂𝐅 ta‐ ble-compression options are specified (see above). That's because if high-performance is most important, one of these options should be used, so if they weren't, 𝗳𝗹𝗲𝘅 assumes it is preferable to trade off a bit of run-time performance for intuitive interactive behavior. Note also that -𝐈 can‐ not be used in conjunction with -𝐂𝗳 or -𝐂𝐅. Thus, this option is not really needed; it is on by default for all those cases in which it is allowed. A scanner can be forced to not be interactive by using -𝐁 (see above). -𝗶 Instructs 𝗳𝗹𝗲𝘅 to generate a case-insensitive scanner. The case of letters given in the 𝗳𝗹𝗲𝘅 input patterns will be ignored, and tokens in the input will be matched regardless of case. The matched text given in y̲y̲t̲e̲x̲t̲ will have the preserved case (i.e., it will not be folded). -𝐋 Instructs 𝗳𝗹𝗲𝘅 not to generate “#line” directives. Without this option, 𝗳𝗹𝗲𝘅 peppers the generated scanner with #line directives so error messages in the actions will be cor‐ rectly located with respect to either the original 𝗳𝗹𝗲𝘅 input file (if the errors are due to code in the input file), or l̲e̲x̲.̲y̲y̲.̲c̲ (if the errors are 𝗳𝗹𝗲𝘅's fault - these sorts of errors should be reported to the email address given below). -𝗹 Turns on maximum compatibility with the original AT&T UNIX 𝗹𝗲𝘅 implementation. Note that this does not mean full com‐ patibility. Use of this option costs a considerable amount of performance, and it cannot be used with the -+, -𝗳, -𝐅, -𝐂𝗳, or -𝐂𝐅 options. For details on the compatibilities it provides, see the section I̲N̲C̲O̲M̲P̲A̲T̲I̲B̲I̲L̲I̲T̲I̲E̲S̲ W̲I̲T̲H̲ L̲E̲X̲ A̲N̲D̲ P̲O̲S̲I̲X̲ below. This option also results in the name YY_FLEX_LEX_COMPAT being #define'd in the generated scan‐ ner. -𝗻 Another do-nothing, deprecated option included only for POSIX compliance. -𝗼o̲u̲t̲p̲u̲t̲ Directs 𝗳𝗹𝗲𝘅 to write the scanner to the file o̲u̲t̲p̲u̲t̲ instead of l̲e̲x̲.̲y̲y̲.̲c̲. If -𝗼 is combined with the -𝘁 option, then the scanner is written to stdout but its “#line” directives (see the -𝐋 option above) refer to the file o̲u̲t̲p̲u̲t̲. -𝐏p̲r̲e̲f̲i̲x̲ Changes the default "yy" prefix used by 𝗳𝗹𝗲𝘅 for all glob‐ ally visible variable and function names to instead be p̲r̲e̲f̲i̲x̲. For example, -𝐏f̲o̲o̲ changes the name of y̲y̲t̲e̲x̲t̲ to f̲o̲o̲t̲e̲x̲t̲. It also changes the name of the default output file from l̲e̲x̲.̲y̲y̲.̲c̲ to l̲e̲x̲.̲f̲o̲o̲.̲c̲. Here are all of the names affected: yy_create_buffer yy_delete_buffer yy_flex_debug yy_init_buffer yy_flush_buffer yy_load_buffer_state yy_switch_to_buffer yyin yyleng yylex yylineno yyout yyrestart yytext yywrap (If using a C++ scanner, then only y̲y̲w̲r̲a̲p̲ and y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲ are affected.) Within the scanner itself, it is still pos‐ sible to refer to the global variables and functions using either version of their name; but externally, they have the modified name. This option allows multiple 𝗳𝗹𝗲𝘅 programs to be easily linked together into the same executable. Note, though, that using this option also renames 𝘆𝘆𝘄𝗿𝗮𝗽(), so now either an (appropriately named) version of the routine for the scanner must be supplied, or “%option noyywrap” must be used, as linking with -𝗹𝗳𝗹 no longer provides one by default. -𝗽 Generates a performance report to stderr. The report con‐ sists of comments regarding features of the 𝗳𝗹𝗲𝘅 input file which will cause a serious loss of performance in the resulting scanner. If the flag is specified twice, com‐ ments regarding features that lead to minor performance losses will also be reported> Note that the use of R̲E̲J̲E̲C̲T̲, “%option yylineno”, and vari‐ able trailing context (see the B̲U̲G̲S̲ section below) entails a substantial performance penalty; use of 𝘆𝘆𝗺𝗼𝗿𝗲(), the ‘^’ operator, and the -𝐈 flag entail minor performance penal‐ ties. -𝐒s̲k̲e̲l̲e̲t̲o̲n̲ Overrides the default skeleton file from which 𝗳𝗹𝗲𝘅 con‐ structs its scanners. This option is needed only for 𝗳𝗹𝗲𝘅 maintenance or development. -𝘀 Causes the default rule (that unmatched scanner input is echoed to stdout) to be suppressed. If the scanner encoun‐ ters input that does not match any of its rules, it aborts with an error. This option is useful for finding holes in a scanner's rule set. -𝐓 Makes 𝗳𝗹𝗲𝘅 run in t̲r̲a̲c̲e̲ mode. It will generate a lot of messages to stderr concerning the form of the input and the resultant non-deterministic and deterministic finite autom‐ ata. This option is mostly for use in maintaining 𝗳𝗹𝗲𝘅. -𝘁 Instructs 𝗳𝗹𝗲𝘅 to write the scanner it generates to stan‐ dard output instead of l̲e̲x̲.̲y̲y̲.̲c̲. -𝐕 Prints the version number to stdout and exits. --𝘃𝗲𝗿𝘀𝗶𝗼𝗻 is a synonym for -𝐕. -𝘃 Specifies that 𝗳𝗹𝗲𝘅 should write to stderr a summary of statistics regarding the scanner it generates. Most of the statistics are meaningless to the casual 𝗳𝗹𝗲𝘅 user, but the first line identifies the version of 𝗳𝗹𝗲𝘅 (same as reported by -𝐕), and the next line the flags used when generating the scanner, including those that are on by default. -𝘄 Suppresses warning messages. -+ Specifies that 𝗳𝗹𝗲𝘅 should generate a C++ scanner class. See the section on G̲E̲N̲E̲R̲A̲T̲I̲N̲G̲ C̲+̲+̲ S̲C̲A̲N̲N̲E̲R̲S̲ below for details. 𝗳𝗹𝗲𝘅 also provides a mechanism for controlling options within the scanner specification itself, rather than from the 𝗳𝗹𝗲𝘅 command line. This is done by including “%option” directives in the first section of the scanner specification. Multiple options can be specified with a single “%option” directive, and multiple direc‐ tives in the first section of the 𝗳𝗹𝗲𝘅 input file. Most options are given simply as names, optionally preceded by the word "no" (with no intervening whitespace) to negate their meaning. A number are equivalent to 𝗳𝗹𝗲𝘅 flags or their negation: 7bit -7 option 8bit -8 option align -Ca option backup -b option batch -B option c++ -+ option caseful or case-sensitive opposite of -i (default) case-insensitive or caseless -i option debug -d option default opposite of -s option ecs -Ce option fast -F option full -f option interactive -I option lex-compat -l option meta-ecs -Cm option perf-report -p option read -Cr option stdout -t option verbose -v option warn opposite of -w option (use "%option nowarn" for -w) array equivalent to "%array" pointer equivalent to "%pointer" (default) Some %option's provide features otherwise not available: always-interactive Instructs 𝗳𝗹𝗲𝘅 to generate a scanner which always considers its input "interactive". Normally, on each new input file the scanner calls 𝗶𝘀𝗮𝘁𝘁𝘆() in an attempt to determine whether the scanner's input source is interactive and thus should be read a character at a time. When this option is used, however, no such call is made. main Directs 𝗳𝗹𝗲𝘅 to provide a default 𝗺𝗮𝗶𝗻() program for the scanner, which simply calls 𝘆𝘆𝗹𝗲𝘅(). This option implies “noyywrap” (see below). never-interactive Instructs 𝗳𝗹𝗲𝘅 to generate a scanner which never considers its input "interactive" (again, no call made to 𝗶𝘀𝗮𝘁𝘁𝘆()). This is the opposite of “always-interactive”. stack Enables the use of start condition stacks (see S̲T̲A̲R̲T̲ C̲O̲N̲D̲I̲T̲I̲O̲N̲S̲ above). stdinit If set (i.e., “%option stdinit”), initializes y̲y̲i̲n̲ and y̲y̲o̲u̲t̲ to stdin and stdout, instead of the default of “nil”. Some existing 𝗹𝗲𝘅 programs depend on this behavior, even though it is not compliant with ANSI C, which does not require stdin and stdout to be compile-time constant. yylineno Directs 𝗳𝗹𝗲𝘅 to generate a scanner that maintains the num‐ ber of the current line read from its input in the global variable y̲y̲l̲i̲n̲e̲n̲o̲. This option is implied by “%option lex-compat”. yywrap If unset (i.e., “%option noyywrap”), makes the scanner not call 𝘆𝘆𝘄𝗿𝗮𝗽() upon an end-of-file, but simply assume that there are no more files to scan (until the user points y̲y̲i̲n̲ at a new file and calls 𝘆𝘆𝗹𝗲𝘅() again). 𝗳𝗹𝗲𝘅 scans rule actions to determine whether the R̲E̲J̲E̲C̲T̲ or 𝘆𝘆𝗺𝗼𝗿𝗲() features are being used. The “reject” and “yymore” options are available to override its decision as to whether to use the options, either by setting them (e.g., “%option reject”) to indi‐ cate the feature is indeed used, or unsetting them to indicate it actually is not used (e.g., “%option noyymore”). Three options take string-delimited values, offset with ‘=’: %option outfile="ABC" is equivalent to -𝗼A̲B̲C̲, and %option prefix="XYZ" is equivalent to -𝐏X̲Y̲Z̲. Finally, %option yyclass="foo" only applies when generating a C++ scanner (-+ option). It informs 𝗳𝗹𝗲𝘅 that “foo” has been derived as a subclass of yyFlexLexer, so 𝗳𝗹𝗲𝘅 will place actions in the member function “foo::yylex()” instead of “yyFlexLexer::yylex()”. It also generates a “yyFlexLexer::yylex()” member function that emits a run-time error (by invoking “yyFlexLexer::LexerError()”) if called. See G̲E̲N̲E̲R̲A̲T̲I̲N̲G̲ C̲+̲+̲ S̲C̲A̲N̲N̲E̲R̲S̲, below, for additional information. A number of options are available for lint purists who want to sup‐ press the appearance of unneeded routines in the generated scanner. Each of the following, if unset (e.g., “%option nounput”), results in the corresponding routine not appearing in the generated scan‐ ner: input, unput yy_push_state, yy_pop_state, yy_top_state yy_scan_buffer, yy_scan_bytes, yy_scan_string (though 𝘆𝘆_𝗽𝘂𝘀𝗵_𝘀𝘁𝗮𝘁𝗲() and friends won't appear anyway unless “%option stack” is being used). 𝐏𝐄𝐑𝐅𝐎𝐑𝐌𝐀𝐍𝐂𝐄 𝐂𝐎𝐍𝐒𝐈𝐃𝐄𝐑𝐀𝐓𝐈𝐎𝐍𝐒 The main design goal of 𝗳𝗹𝗲𝘅 is that it generate high-performance scanners. It has been optimized for dealing well with large sets of rules. Aside from the effects on scanner speed of the table compression -𝐂 options outlined above, there are a number of options/actions which degrade performance. These are, from most expensive to least: REJECT %option yylineno arbitrary trailing context pattern sets that require backing up %array %option interactive %option always-interactive '^' beginning-of-line operator yymore() with the first three all being quite expensive and the last two being quite cheap. Note also that 𝘂𝗻𝗽𝘂𝘁() is implemented as a rou‐ tine call that potentially does quite a bit of work, while 𝘆𝘆𝗹𝗲𝘀𝘀() is a quite-cheap macro; so if just putting back some excess text, use 𝘆𝘆𝗹𝗲𝘀𝘀(). R̲E̲J̲E̲C̲T̲ should be avoided at all costs when performance is impor‐ tant. It is a particularly expensive option. Getting rid of backing up is messy and often may be an enormous amount of work for a complicated scanner. In principal, one begins by using the -𝗯 flag to generate a l̲e̲x̲.̲b̲a̲c̲k̲u̲p̲ file. For example, on the input %% foo return TOK_KEYWORD; foobar return TOK_KEYWORD; the file looks like: State #6 is non-accepting - associated rule line numbers: 2 3 out-transitions: [ o ] jam-transitions: EOF [ \001-n p-\177 ] State #8 is non-accepting - associated rule line numbers: 3 out-transitions: [ a ] jam-transitions: EOF [ \001-` b-\177 ] State #9 is non-accepting - associated rule line numbers: 3 out-transitions: [ r ] jam-transitions: EOF [ \001-q s-\177 ] Compressed tables always back up. The first few lines tell us that there's a scanner state in which it can make a transition on an ‘o’ but not on any other character, and that in that state the currently scanned text does not match any rule. The state occurs when trying to match the rules found at lines 2 and 3 in the input file. If the scanner is in that state and then reads something other than an ‘o’, it will have to back up to find a rule which is matched. With a bit of headscratching one can see that this must be the state it's in when it has seen ‘fo’. When this has happened, if anything other than another ‘o’ is seen, the scanner will have to back up to simply match the ‘f’ (by the default rule). The comment regarding State #8 indicates there's a problem when "foob" has been scanned. Indeed, on any character other than an ‘a’, the scanner will have to back up to accept "foo". Similarly, the comment for State #9 concerns when "fooba" has been scanned and an ‘r’ does not follow. The final comment reminds us that there's no point going to all the trouble of removing backing up from the rules unless we're using -𝐂𝗳 or -𝐂𝐅, since there's no performance gain doing so with com‐ pressed scanners. The way to remove the backing up is to add "error" rules: %% foo return TOK_KEYWORD; foobar return TOK_KEYWORD; fooba | foob | fo { /* false alarm, not really a keyword */ return TOK_ID; } Eliminating backing up among a list of keywords can also be done using a "catch-all" rule: %% foo return TOK_KEYWORD; foobar return TOK_KEYWORD; [a-z]+ return TOK_ID; This is usually the best solution when appropriate. Backing up messages tend to cascade. With a complicated set of rules it's not uncommon to get hundreds of messages. If one can decipher them, though, it often only takes a dozen or so rules to eliminate the backing up (though it's easy to make a mistake and have an error rule accidentally match a valid token; a possible future 𝗳𝗹𝗲𝘅 feature will be to automatically add rules to eliminate backing up). It's important to keep in mind that the benefits of eliminating backing up are gained only if e̲v̲e̲r̲y̲ instance of backing up is elim‐ inated. Leaving just one gains nothing. V̲a̲r̲i̲a̲b̲l̲e̲ trailing context (where both the leading and trailing parts do not have a fixed length) entails almost the same perfor‐ mance loss as R̲E̲J̲E̲C̲T̲ (i.e., substantial). So when possible a rule like: %% mouse|rat/(cat|dog) run(); is better written: %% mouse/cat|dog run(); rat/cat|dog run(); or as %% mouse|rat/cat run(); mouse|rat/dog run(); Note that here the special ‘|’ action does not provide any savings, and can even make things worse (see B̲U̲G̲S̲ below). Another area where the user can increase a scanner's performance (and one that's easier to implement) arises from the fact that the longer the tokens matched, the faster the scanner will run. This is because with long tokens the processing of most input characters takes place in the (short) inner scanning loop, and does not often have to go through the additional work of setting up the scanning environment (e.g., y̲y̲t̲e̲x̲t̲) for the action. Recall the scanner for C comments: %x comment %% int line_num = 1; "/*" BEGIN(comment); [^*\n]* "*"+[^*/\n]* \n ++line_num; "*"+"/" BEGIN(INITIAL); This could be sped up by writing it as: %x comment %% int line_num = 1; "/*" BEGIN(comment); [^*\n]* [^*\n]*\n ++line_num; "*"+[^*/\n]* "*"+[^*/\n]*\n ++line_num; "*"+"/" BEGIN(INITIAL); Now instead of each newline requiring the processing of another action, recognizing the newlines is "distributed" over the other rules to keep the matched text as long as possible. Note that adding rules does n̲o̲t̲ slow down the scanner! The speed of the scanner is independent of the number of rules or (modulo the con‐ siderations given at the beginning of this section) how complicated the rules are with regard to operators such as ‘*’ and ‘|’. A final example in speeding up a scanner: scan through a file con‐ taining identifiers and keywords, one per line and with no other extraneous characters, and recognize all the keywords. A natural first approach is: %% asm | auto | break | ... etc ... volatile | while /* it's a keyword */ .|\n /* it's not a keyword */ To eliminate the back-tracking, introduce a catch-all rule: %% asm | auto | break | ... etc ... volatile | while /* it's a keyword */ [a-z]+ | .|\n /* it's not a keyword */ Now, if it's guaranteed that there's exactly one word per line, then we can reduce the total number of matches by a half by merging in the recognition of newlines with that of the other tokens: %% asm\n | auto\n | break\n | ... etc ... volatile\n | while\n /* it's a keyword */ [a-z]+\n | .|\n /* it's not a keyword */ One has to be careful here, as we have now reintroduced backing up into the scanner. In particular, while we know that there will never be any characters in the input stream other than letters or newlines, 𝗳𝗹𝗲𝘅 can't figure this out, and it will plan for possibly needing to back up when it has scanned a token like "auto" and then the next character is something other than a newline or a letter. Previously it would then just match the "auto" rule and be done, but now it has no "auto" rule, only an "auto\n" rule. To eliminate the possibility of backing up, we could either duplicate all rules but without final newlines or, since we never expect to encounter such an input and therefore don't how it's classified, we can introduce one more catch-all rule, this one which doesn't include a newline: %% asm\n | auto\n | break\n | ... etc ... volatile\n | while\n /* it's a keyword */ [a-z]+\n | [a-z]+ | .|\n /* it's not a keyword */ Compiled with -𝐂𝗳, this is about as fast as one can get a 𝗳𝗹𝗲𝘅 scanner to go for this particular problem. A final note: 𝗳𝗹𝗲𝘅 is slow when matching NUL's, particularly when a token contains multiple NUL's. It's best to write rules which match short amounts of text if it's anticipated that the text will often include NUL's. Another final note regarding performance: as mentioned above in the section H̲O̲W̲ T̲H̲E̲ I̲N̲P̲U̲T̲ I̲S̲ M̲A̲T̲C̲H̲E̲D̲, dynamically resizing y̲y̲t̲e̲x̲t̲ to accommodate huge tokens is a slow process because it presently requires that the (huge) token be rescanned from the beginning. Thus if performance is vital, it is better to attempt to match "large" quantities of text but not "huge" quantities, where the cutoff between the two is at about 8K characters/token. 𝐆𝐄𝐍𝐄𝐑𝐀𝐓𝐈𝐍𝐆 𝐂++ 𝐒𝐂𝐀𝐍𝐍𝐄𝐑𝐒 𝗳𝗹𝗲𝘅 provides two different ways to generate scanners for use with C++. The first way is to simply compile a scanner generated by 𝗳𝗹𝗲𝘅 using a C++ compiler instead of a C compiler. This should not generate any compilation errors (please report any found to the email address given in the A̲U̲T̲H̲O̲R̲S̲ section below). C++ code can then be used in rule actions instead of C code. Note that the default input source for scanners remains y̲y̲i̲n̲, and default echoing is still done to y̲y̲o̲u̲t̲. Both of these remain F̲I̲L̲E̲ *̲ variables and not C++ streams. 𝗳𝗹𝗲𝘅 can also be used to generate a C++ scanner class, using the -+ option (or, equivalently, “%option c++”), which is automatically specified if the name of the flex executable ends in a ‘+’, such as 𝗳𝗹𝗲𝘅++. When using this option, 𝗳𝗹𝗲𝘅 defaults to generating the scanner to the file l̲e̲x̲.̲y̲y̲.̲c̲c̲ instead of l̲e̲x̲.̲y̲y̲.̲c̲. The generated scanner includes the header file , which defines the interface to two C++ classes. The first class, F̲l̲e̲x̲L̲e̲x̲e̲r̲, provides an abstract base class defin‐ ing the general scanner class interface. It provides the following member functions: const char* YYText() Returns the text of the most recently matched token, the equivalent of y̲y̲t̲e̲x̲t̲. int YYLeng() Returns the length of the most recently matched token, the equivalent of y̲y̲l̲e̲n̲g̲. int lineno() const Returns the current input line number (see “%option yylineno”), or 1 if “%option yylineno” was not used. void set_debug(int flag) Sets the debugging flag for the scanner, equivalent to assigning to y̲y̲_f̲l̲e̲x̲_d̲e̲b̲u̲g̲ (see the O̲P̲T̲I̲O̲N̲S̲ section above). Note that the scanner must be built using “%option debug” to include debugging information in it. int debug() const Returns the current setting of the debugging flag. Also provided are member functions equivalent to 𝘆𝘆_𝘀𝘄𝗶𝘁𝗰𝗵_𝘁𝗼_𝗯𝘂𝗳𝗳𝗲𝗿(), 𝘆𝘆_𝗰𝗿𝗲𝗮𝘁𝗲_𝗯𝘂𝗳𝗳𝗲𝗿() (though the first argu‐ ment is an s̲t̲d̲:̲:̲i̲s̲t̲r̲e̲a̲m̲*̲ object pointer and not a F̲I̲L̲E̲*̲), 𝘆𝘆_𝗳𝗹𝘂𝘀𝗵_𝗯𝘂𝗳𝗳𝗲𝗿(), 𝘆𝘆_𝗱𝗲𝗹𝗲𝘁𝗲_𝗯𝘂𝗳𝗳𝗲𝗿(), and 𝘆𝘆𝗿𝗲𝘀𝘁𝗮𝗿𝘁() (again, the first argument is an s̲t̲d̲:̲:̲i̲s̲t̲r̲e̲a̲m̲*̲ object pointer). The second class defined in is y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲, which is derived from F̲l̲e̲x̲L̲e̲x̲e̲r̲. It defines the following additional member functions: yyFlexLexer(std::istream* arg_yyin = 0, std::ostream* arg_yyout = 0) Constructs a y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲ object using the given streams for input and output. If not specified, the streams default to c̲i̲n̲ and c̲o̲u̲t̲, respectively. virtual int yylex() Performs the same role as 𝘆𝘆𝗹𝗲𝘅() does for ordinary flex scanners: it scans the input stream, consuming tokens, until a rule's action returns a value. If subclass ‘S’ is derived from y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲, in order to access the member functions and variables of ‘S’ inside 𝘆𝘆𝗹𝗲𝘅(), use “%option yyclass="S"” to inform 𝗳𝗹𝗲𝘅 that the ‘S’ subclass will be used instead of y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲. In this case, rather than generating “yyFlexLexer::yylex()”, 𝗳𝗹𝗲𝘅 generates “S::yylex()” (and also generates a dummy “yyFlexLexer::yylex()” that calls “yyFlexLexer::LexerError()” if called). virtual void switch_streams(std::istream* new_in = 0, std::ostream* new_out = 0) Reassigns y̲y̲i̲n̲ to n̲e̲w̲_i̲n̲ (if non-nil) and y̲y̲o̲u̲t̲ to n̲e̲w̲_o̲u̲t̲ (ditto), deleting the previous input buffer if y̲y̲i̲n̲ is reassigned. int yylex(std::istream* new_in, std::ostream* new_out = 0) First switches the input streams via “switch_streams(new_in, new_out)” and then returns the value of 𝘆𝘆𝗹𝗲𝘅(). In addition, y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲ defines the following protected virtual functions which can be redefined in derived classes to tailor the scanner: virtual int LexerInput(char* buf, int max_size) Reads up to m̲a̲x̲_s̲i̲z̲e̲ characters into b̲u̲f̲ and returns the number of characters read. To indicate end-of-input, return 0 characters. Note that "interactive" scanners (see the -𝐁 and -𝐈 flags) define the macro YY_INTERACTIVE. If 𝐋𝗲𝘅𝗲𝗿𝐈𝗻𝗽𝘂𝘁() has been redefined, and it's necessary to take different actions depending on whether or not the scanner might be scanning an interactive input source, it's possi‐ ble to test for the presence of this name via “#ifdef”. virtual void LexerOutput(const char* buf, int size) Writes out s̲i̲z̲e̲ characters from the buffer b̲u̲f̲, which, while NUL-terminated, may also contain "internal" NUL's if the scanner's rules can match text with NUL's in them. virtual void LexerError(const char* msg) Reports a fatal error message. The default version of this function writes the message to the stream c̲e̲r̲r̲ and exits. Note that a y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲ object contains its entire scanning state. Thus such objects can be used to create reentrant scanners. Multi‐ ple instances of the same y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲ class can be instantiated, and multiple C++ scanner classes can be combined in the same pro‐ gram using the -𝐏 option discussed above. Finally, note that the “%array” feature is not available to C++ scanner classes; “%pointer” must be used (the default). Here is an example of a simple C++ scanner: // An example of using the flex C++ scanner class. %{ #include int mylineno = 0; %} string \"[^\n"]+\" ws [ \t]+ alpha [A-Za-z] dig [0-9] name ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])* num1 [-+]?{dig}+\.?([eE][-+]?{dig}+)? num2 [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)? number {num1}|{num2} %% {ws} /* skip blanks and tabs */ "/*" { int c; while ((c = yyinput()) != 0) { if(c == '\n') ++mylineno; else if(c == '*') { if ((c = yyinput()) == '/') break; else unput(c); } } } {number} cout << "number " << YYText() << '\n'; \n mylineno++; {name} cout << "name " << YYText() << '\n'; {string} cout << "string " << YYText() << '\n'; %% int main(int /* argc */, char** /* argv */) { FlexLexer* lexer = new yyFlexLexer; while(lexer->yylex() != 0) ; return 0; } To create multiple (different) lexer classes, use the -𝐏 flag (or the “prefix=” option) to rename each y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲ to some other x̲x̲F̲l̲e̲x̲L̲e̲x̲e̲r̲. can then be included in other sources once per lexer class, first renaming y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲ as fol‐ lows: #undef yyFlexLexer #define yyFlexLexer xxFlexLexer #include #undef yyFlexLexer #define yyFlexLexer zzFlexLexer #include If, for example, “%option prefix="xx"” is used for one scanner and “%option prefix="zz"” is used for the other. 𝐈𝐌𝐏𝐎𝐑𝐓𝐀𝐍𝐓: the present form of the scanning class is experimental and may change considerably between major releases. 𝐈𝐍𝐂𝐎𝐌𝐏𝐀𝐓𝐈𝐁𝐈𝐋𝐈𝐓𝐈𝐄𝐒 𝐖𝐈𝐓𝐇 𝐋𝐄𝐗 𝐀𝐍𝐃 𝐏𝐎𝐒𝐈𝐗 𝗳𝗹𝗲𝘅 is a rewrite of the AT&T UNIX 𝗹𝗲𝘅 tool (the two implementa‐ tions do not share any code, though), with some extensions and incompatibilities, both of which are of concern to those who wish to write scanners acceptable to either implementation. 𝗳𝗹𝗲𝘅 is fully compliant with the POSIX 𝗹𝗲𝘅 specification, except that when using “%pointer” (the default), a call to 𝘂𝗻𝗽𝘂𝘁() destroys the con‐ tents of y̲y̲t̲e̲x̲t̲, which is counter to the POSIX specification. In this section we discuss all of the known areas of incompatibil‐ ity between 𝗳𝗹𝗲𝘅, AT&T UNIX 𝗹𝗲𝘅, and the POSIX specification. 𝗳𝗹𝗲𝘅's -𝗹 option turns on maximum compatibility with the original AT&T UNIX 𝗹𝗲𝘅 implementation, at the cost of a major loss in the generated scanner's performance. We note below which incompatibil‐ ities can be overcome using the -𝗹 option. 𝗳𝗹𝗲𝘅 is fully compatible with 𝗹𝗲𝘅 with the following exceptions: - The undocumented 𝗹𝗲𝘅 scanner internal variable y̲y̲l̲i̲n̲e̲n̲o̲ is not supported unless -𝗹 or “%option yylineno” is used. y̲y̲l̲i̲n̲e̲n̲o̲ should be maintained on a per-buffer basis, rather than a per-scanner (single global variable) basis. y̲y̲l̲i̲n̲e̲n̲o̲ is not part of the POSIX specification. - The 𝗶𝗻𝗽𝘂𝘁() routine is not redefinable, though it may be called to read characters following whatever has been matched by a rule. If 𝗶𝗻𝗽𝘂𝘁() encounters an end-of-file, the normal 𝘆𝘆𝘄𝗿𝗮𝗽() processing is done. A “real” end-of-file is returned by 𝗶𝗻𝗽𝘂𝘁() as EOF. Input is instead controlled by defining the YY_INPUT macro. The 𝗳𝗹𝗲𝘅 restriction that 𝗶𝗻𝗽𝘂𝘁() cannot be redefined is in accordance with the POSIX specification, which simply does not specify any way of controlling the scanner's input other than by making an initial assignment to y̲y̲i̲n̲. - The 𝘂𝗻𝗽𝘂𝘁() routine is not redefinable. This restriction is in accordance with POSIX. - 𝗳𝗹𝗲𝘅 scanners are not as reentrant as 𝗹𝗲𝘅 scanners. In partic‐ ular, if a scanner is interactive and an interrupt handler long-jumps out of the scanner, and the scanner is subsequently called again, the following error message may be displayed: fatal flex scanner internal error--end of buffer missed To reenter the scanner, first use yyrestart(yyin); Note that this call will throw away any buffered input; usually this isn't a problem with an interactive scanner. Also note that flex C++ scanner classes are reentrant, so if using C++ is an option , they should be used instead. See G̲E̲N̲E̲R̲A̲T̲I̲N̲G̲ C̲+̲+̲ S̲C̲A̲N̲N̲E̲R̲S̲ above for details. - 𝗼𝘂𝘁𝗽𝘂𝘁() is not supported. Output from the E̲C̲H̲O̲ macro is done to the file-pointer y̲y̲o̲u̲t̲ (default stdout). 𝗼𝘂𝘁𝗽𝘂𝘁() is not part of the POSIX specification. - 𝗹𝗲𝘅 does not support exclusive start conditions (%x), though they are in the POSIX specification. - When definitions are expanded, 𝗳𝗹𝗲𝘅 encloses them in parenthe‐ ses. With 𝗹𝗲𝘅, the following: NAME [A-Z][A-Z0-9]* %% foo{NAME}? printf("Found it\n"); %% will not match the string "foo" because when the macro is expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the precedence is such that the ‘?’ is associated with "[A-Z0-9]*". With 𝗳𝗹𝗲𝘅, the rule will be expanded to "foo([A-Z][A-Z0-9]*)?" and so the string "foo" will match. Note that if the definition begins with ‘^’ or ends with ‘$’ then it is not expanded with parentheses, to allow these opera‐ tors to appear in definitions without losing their special meanings. But the ‘⟨s⟩’, ‘/’, and ⟨⟨EOF⟩⟩ operators cannot be used in a 𝗳𝗹𝗲𝘅 definition. Using -𝗹 results in the 𝗹𝗲𝘅 behavior of no parentheses around the definition. The POSIX specification is that the definition be enclosed in parentheses. - Some implementations of 𝗹𝗲𝘅 allow a rule's action to begin on a separate line, if the rule's pattern has trailing whitespace: %% foo|bar { foobar_action(); } 𝗳𝗹𝗲𝘅 does not support this feature. - The 𝗹𝗲𝘅 ‘%r’ (generate a Ratfor scanner) option is not sup‐ ported. It is not part of the POSIX specification. - After a call to 𝘂𝗻𝗽𝘂𝘁(), y̲y̲t̲e̲x̲t̲ is undefined until the next token is matched, unless the scanner was built using “%array”. This is not the case with 𝗹𝗲𝘅 or the POSIX specification. The -𝗹 option does away with this incompatibility. - The precedence of the ‘{}’ (numeric range) operator is differ‐ ent. 𝗹𝗲𝘅 interprets "abc{1,3}" as match one, two, or three occurrences of ‘abc’, whereas 𝗳𝗹𝗲𝘅 interprets it as match ‘ab’ followed by one, two, or three occurrences of ‘c’. The latter is in agreement with the POSIX specification. - The precedence of the ‘^’ operator is different. 𝗹𝗲𝘅 inter‐ prets "^foo|bar" as match either ‘foo’ at the beginning of a line, or ‘bar’ anywhere, whereas 𝗳𝗹𝗲𝘅 interprets it as match either ‘foo’ or ‘bar’ if they come at the beginning of a line. The latter is in agreement with the POSIX specification. - The special table-size declarations such as ‘%a’ supported by 𝗹𝗲𝘅 are not required by 𝗳𝗹𝗲𝘅 scanners; 𝗳𝗹𝗲𝘅 ignores them. - The name FLEX_SCANNER is #define'd so scanners may be written for use with either 𝗳𝗹𝗲𝘅 or 𝗹𝗲𝘅. Scanners also include YY_FLEX_MAJOR_VERSION and YY_FLEX_MINOR_VERSION indicating which version of 𝗳𝗹𝗲𝘅 generated the scanner (for example, for the 2.5 release, these defines would be 2 and 5, respectively). The following 𝗳𝗹𝗲𝘅 features are not included in 𝗹𝗲𝘅 or the POSIX specification: C++ scanners %option start condition scopes start condition stacks interactive/non-interactive scanners yy_scan_string() and friends yyterminate() yy_set_interactive() yy_set_bol() YY_AT_BOL() <> <*> YY_DECL YY_START YY_USER_ACTION YY_USER_INIT #line directives %{}'s around actions multiple actions on a line plus almost all of the 𝗳𝗹𝗲𝘅 flags. The last feature in the list refers to the fact that with 𝗳𝗹𝗲𝘅 multiple actions can be placed on the same line, separated with semi-colons, while with 𝗹𝗲𝘅, the fol‐ lowing foo handle_foo(); ++num_foos_seen; is (rather surprisingly) truncated to foo handle_foo(); 𝗳𝗹𝗲𝘅 does not truncate the action. Actions that are not enclosed in braces are simply terminated at the end of the line. 𝐅𝐈𝐋𝐄𝐒 flex.skl Skeleton scanner. This file is only used when building flex, not when 𝗳𝗹𝗲𝘅 executes. lex.backup Backing-up information for the -𝗯 flag (called l̲e̲x̲.̲b̲c̲k̲ on some systems). lex.yy.c Generated scanner (called l̲e̲x̲y̲y̲.̲c̲ on some sys‐ tems). lex.yy.cc Generated C++ scanner class, when using -+. Header file defining the C++ scanner base class, F̲l̲e̲x̲L̲e̲x̲e̲r̲, and its derived class, y̲y̲F̲l̲e̲x̲L̲e̲x̲e̲r̲. /usr/lib/libl.* 𝗳𝗹𝗲𝘅 libraries. The /̲u̲s̲r̲/̲l̲i̲b̲/̲l̲i̲b̲f̲l̲.̲*̲ libraries are links to these. Scanners must be linked using either -𝗹𝗹 or -𝗹𝗳𝗹. 𝐄𝐗𝐈𝐓 𝐒𝐓𝐀𝐓𝐔𝐒 The 𝗳𝗹𝗲𝘅 utility exits 0 on success, and >0 if an error occurs. 𝐃𝐈𝐀𝐆𝐍𝐎𝐒𝐓𝐈𝐂𝐒 𝘄𝗮𝗿𝗻𝗶𝗻𝗴, 𝗿𝘂𝗹𝗲 𝗰𝗮𝗻𝗻𝗼𝘁 𝗯𝗲 𝗺𝗮𝘁𝗰𝗵𝗲𝗱 Indicates that the given rule can‐ not be matched because it follows other rules that will always match the same text as it. For example, in the following “foo” cannot be matched because it comes after an identifier "catch-all" rule: [a-z]+ got_identifier(); foo got_foo(); Using R̲E̲J̲E̲C̲T̲ in a scanner suppresses this warning. 𝘄𝗮𝗿𝗻𝗶𝗻𝗴, -𝘀 𝗼𝗽𝘁𝗶𝗼𝗻 𝗴𝗶𝘃𝗲𝗻 𝗯𝘂𝘁 𝗱𝗲𝗳𝗮𝘂𝗹𝘁 𝗿𝘂𝗹𝗲 𝗰𝗮𝗻 𝗯𝗲 𝗺𝗮𝘁𝗰𝗵𝗲𝗱 Means that it is possible (perhaps only in a particular start condition) that the default rule (match any single character) is the only one that will match a particular input. Since -𝘀 was given, presumably this is not intended. 𝗿𝗲𝗷𝗲𝗰𝘁_𝘂𝘀𝗲𝗱_𝗯𝘂𝘁_𝗻𝗼𝘁_𝗱𝗲𝘁𝗲𝗰𝘁𝗲𝗱 𝘂𝗻𝗱𝗲𝗳𝗶𝗻𝗲𝗱 𝘆𝘆𝗺𝗼𝗿𝗲_𝘂𝘀𝗲𝗱_𝗯𝘂𝘁_𝗻𝗼𝘁_𝗱𝗲𝘁𝗲𝗰𝘁𝗲𝗱 𝘂𝗻𝗱𝗲𝗳𝗶𝗻𝗲𝗱 These errors can occur at compile time. They indicate that the scanner uses R̲E̲J̲E̲C̲T̲ or 𝘆𝘆𝗺𝗼𝗿𝗲() but that 𝗳𝗹𝗲𝘅 failed to notice the fact, meaning that 𝗳𝗹𝗲𝘅 scanned the first two sections looking for occurrences of these actions and failed to find any, but somehow they snuck in (via an #include file, for example). Use “%option reject” or “%option yymore” to indicate to 𝗳𝗹𝗲𝘅 that these features are really needed. 𝗳𝗹𝗲𝘅 𝘀𝗰𝗮𝗻𝗻𝗲𝗿 𝗷𝗮𝗺𝗺𝗲𝗱 A scanner compiled with -𝘀 has encountered an input string which wasn't matched by any of its rules. This error can also occur due to internal problems. 𝘁𝗼𝗸𝗲𝗻 𝘁𝗼𝗼 𝗹𝗮𝗿𝗴𝗲, 𝗲𝘅𝗰𝗲𝗲𝗱𝘀 𝐘𝐘𝐋𝐌𝐀𝐗 The scanner uses “%array” and one of its rules matched a string longer than the YYLMAX constant (8K bytes by default). The value can be increased by #define'ing YYLMAX in the definitions section of 𝗳𝗹𝗲𝘅 input. 𝘀𝗰𝗮𝗻𝗻𝗲𝗿 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝘀 -𝟴 𝗳𝗹𝗮𝗴 𝘁𝗼 𝘂𝘀𝗲 𝘁𝗵𝗲 𝗰𝗵𝗮𝗿𝗮𝗰𝘁𝗲𝗿 '𝘅' The scanner specification includes recognizing the 8-bit character ‘x’ and the -𝟴 flag was not specified, and defaulted to 7-bit because the -𝐂𝗳 or -𝐂𝐅 table compression options were used. See the discussion of the -𝟳 flag for details. 𝗳𝗹𝗲𝘅 𝘀𝗰𝗮𝗻𝗻𝗲𝗿 𝗽𝘂𝘀𝗵-𝗯𝗮𝗰𝗸 𝗼𝘃𝗲𝗿𝗳𝗹𝗼𝘄 unput() was used to push back so much text that the scanner's buffer could not hold both the pushed- back text and the current token in y̲y̲t̲e̲x̲t̲. Ideally the scanner should dynamically resize the buffer in this case, but at present it does not. 𝗶𝗻𝗽𝘂𝘁 𝗯𝘂𝗳𝗳𝗲𝗿 𝗼𝘃𝗲𝗿𝗳𝗹𝗼𝘄, 𝗰𝗮𝗻'𝘁 𝗲𝗻𝗹𝗮𝗿𝗴𝗲 𝗯𝘂𝗳𝗳𝗲𝗿 𝗯𝗲𝗰𝗮𝘂𝘀𝗲 𝘀𝗰𝗮𝗻𝗻𝗲𝗿 𝘂𝘀𝗲𝘀 𝐑𝐄𝐉𝐄𝐂𝐓 The scanner was working on matching an extremely large token and needed to expand the input buffer. This doesn't work with scanners that use R̲E̲J̲E̲C̲T̲. 𝗳𝗮𝘁𝗮𝗹 𝗳𝗹𝗲𝘅 𝘀𝗰𝗮𝗻𝗻𝗲𝗿 𝗶𝗻𝘁𝗲𝗿𝗻𝗮𝗹 𝗲𝗿𝗿𝗼𝗿--𝗲𝗻𝗱 𝗼𝗳 𝗯𝘂𝗳𝗳𝗲𝗿 𝗺𝗶𝘀𝘀𝗲𝗱 This can occur in an scanner which is reentered after a long-jump has jumped out (or over) the scanner's activation frame. Before reentering the scanner, use: yyrestart(yyin); or, as noted above, switch to using the C++ scanner class. 𝘁𝗼𝗼 𝗺𝗮𝗻𝘆 𝘀𝘁𝗮𝗿𝘁 𝗰𝗼𝗻𝗱𝗶𝘁𝗶𝗼𝗻𝘀 𝗶𝗻 <> 𝗰𝗼𝗻𝘀𝘁𝗿𝘂𝗰𝘁! More start conditions than exist were listed in a <> construct (so at least one of them must have been listed twice). 𝐒𝐄𝐄 𝐀𝐋𝐒𝐎 awk(1), sed(1), yacc(1) John Levine, Tony Mason, and Doug Brown, L̲e̲x̲ &̲ Y̲a̲c̲c̲, O̲'̲R̲e̲i̲l̲l̲y̲ a̲n̲d̲ A̲s̲s̲o̲c̲i̲a̲t̲e̲s̲, 2nd edition. Alfred Aho, Ravi Sethi, and Jeffrey Ullman, C̲o̲m̲p̲i̲l̲e̲r̲s̲:̲ P̲r̲i̲n̲c̲i̲p̲l̲e̲s̲,̲ T̲e̲c̲h̲n̲i̲q̲u̲e̲s̲ a̲n̲d̲ T̲o̲o̲l̲s̲, A̲d̲d̲i̲s̲o̲n̲-̲W̲e̲s̲l̲e̲y̲, 1986, Describes the pattern- matching techniques used by flex (deterministic finite automata). 𝐒𝐓𝐀𝐍𝐃𝐀𝐑𝐃𝐒 The 𝗹𝗲𝘅 utility is compliant with the IEEE Std 1003.1-2008 (“POSIX.1”) specification, though its presence is optional. The flags [-𝟳𝟴𝐁𝗯𝐂𝗱𝐅𝗳𝗵𝐈𝗶𝐋𝗹𝗼𝐏𝗽𝐒𝘀𝐓𝐕𝘄+?], [--𝗵𝗲𝗹𝗽], and [--𝘃𝗲𝗿𝘀𝗶𝗼𝗻] are extensions to that specification. See also the I̲N̲C̲O̲M̲P̲A̲T̲I̲B̲I̲L̲I̲T̲I̲E̲S̲ W̲I̲T̲H̲ L̲E̲X̲ A̲N̲D̲ P̲O̲S̲I̲X̲ section, above. 𝐀𝐔𝐓𝐇𝐎𝐑𝐒 Vern Paxson, with the help of many ideas and much inspiration from Van Jacobson. Original version by Jef Poskanzer. The fast table representation is a partial implementation of a design done by Van Jacobson. The implementation was done by Kevin Gong and Vern Pax‐ son. Thanks to the many 𝗳𝗹𝗲𝘅 beta-testers, feedbackers, and contribu‐ tors, especially Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Adermann, Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, Nelson H.F. Beebe, b̲e̲n̲s̲o̲n̲@̲o̲d̲i̲.̲c̲o̲m̲, Karl Berry, Peter A. Bigot, Simon Blanchard, Keith Bostic, Frederic Brehm, Ian Brock‐ bank, Kin Cho, Nick Christopher, Brian Clapper, J.T. Conklin, Jason Coughlin, Bill Cox, Nick Cropper, Dave Curtis, Scott David Daniels, Chris G. Demetriou, Theo de Raadt, Mike Donahue, Chuck Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey Friedl, Joe Gayda, Kaveh R. Ghazi, Wolfgang Glunz, Eric Goldman, Christopher M. Gould, Ulrich Grepel, Peer Griebel, Jan Hajic, Charles Hemphill, NORO Hideo, Jarkko Hietaniemi, Scott Hof‐ mann, Jeff Honig, Dana Hudes, Eric Hughes, John Interrante, Ceriel Jacobs, Michal Jaegermann, Sakari Jalovaara, Jeffrey R. Jones, Henry Juengst, Klaus Kaempf, Jonathan I. Kamens, Terrence O Kane, Amir Katz, k̲e̲n̲@̲k̲e̲n̲.̲h̲i̲l̲c̲o̲.̲c̲o̲m̲, Kevin B. Kenny, Steve Kirsch, Win‐ fried Koenig, Marq Kole, Ronald Lamprecht, Greg Lee, Rohan Lenard, Craig Leres, John Levine, Steve Liddle, David Loffredo, Mike Long, Mohamed el Lozy, Brian Madsen, Malte, Joe Marshall, Bengt Martens‐ son, Chris Metcalf, Luke Mewburn, Jim Meyering, R. Alexander Milowski, Erik Naggum, G.T. Nicol, Landon Noll, James Nordby, Marc Nozell, Richard Ohnemus, Karsten Pahnke, Sven Panne, Roland Pesch, Walter Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh, Jarmo Raiha, Frederic Raimbault, Pat Rankin, Rick Richard‐ son, Kevin Rodgers, Kai Uwe Rommel, Jim Roskind, Alberto Santini, Andreas Scherer, Darrell Schiebel, Raf Schietekat, Doug Schmidt, Philippe Schnoebelen, Andreas Schwab, Larry Schwimmer, Alex Siegel, Eckehard Stolz, Jan-Erik Strvmquist, Mike Stump, Paul Stuart, Dave Tallman, Ian Lance Taylor, Chris Thewalt, Richard M. Timoney, Jodi Tsai, Paul Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken Yap, Ron Zellar, Nathan Zelle, David Zuhn, and those whose names have slipped my marginal mail-archiving skills but whose contributions are appreciated all the same. Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John Gilmore, Craig Leres, John Levine, Bob Mulcahy, G.T. Nicol, Francois Pinard, Rich Salz, and Richard Stallman for help with various dis‐ tribution headaches. Thanks to Esmond Pitt and Earle Horton for 8-bit character support; to Benson Margulies and Fred Burke for C++ support; to Kent Williams and Tom Epperly for C++ class support; to Ove Ewerlid for support of NUL's; and to Eric Hughes for support of multiple buf‐ fers. This work was primarily done when I was with the Real Time Systems Group at the Lawrence Berkeley Laboratory in Berkeley, CA. Many thanks to all there for the support I received. Send comments to ⟨v̲e̲r̲n̲@̲e̲e̲.̲l̲b̲l̲.̲g̲o̲v̲⟩. 𝐁𝐔𝐆𝐒 Some trailing context patterns cannot be properly matched and gen‐ erate warning messages (dangerous trailing context). These are patterns where the ending of the first part of the rule matches the beginning of the second part, such as "zx*/xy*", where the ‘x*’ matches the ‘x’ at the beginning of the trailing context. (Note that the POSIX draft states that the text matched by such patterns is undefined.) For some trailing context rules, parts which are actually fixed- length are not recognized as such, leading to the above mentioned performance loss. In particular, parts using ‘|’ or ‘{n}’ (such as "foo{3}") are always considered variable-length. Combining trailing context with the special ‘|’ action can result in fixed trailing context being turned into the more expensive variable trailing context. For example, in the following: %% abc | xyz/def Use of 𝘂𝗻𝗽𝘂𝘁() invalidates yytext and yyleng, unless the “%array” directive or the -𝗹 option has been used. Pattern-matching of NUL's is substantially slower than matching other characters. Dynamic resizing of the input buffer is slow, as it entails rescan‐ ning all the text matched so far by the current (generally huge) token. Due to both buffering of input and read-ahead, it is not possible to intermix calls to routines, such as, for example, 𝗴𝗲𝘁𝗰𝗵𝗮𝗿(), with 𝗳𝗹𝗲𝘅 rules and expect it to work. Call 𝗶𝗻𝗽𝘂𝘁() instead. The total table entries listed by the -𝘃 flag excludes the number of table entries needed to determine what rule has been matched. The number of entries is equal to the number of DFA states if the scanner does not use R̲E̲J̲E̲C̲T̲, and somewhat greater than the number of states if it does. R̲E̲J̲E̲C̲T̲ cannot be used with the -𝗳 or -𝐅 options. The 𝗳𝗹𝗲𝘅 internal algorithms need documentation. COSMOPOLITAN September 21, 2015 BSD