FLEX(1)

💅 FLEX(1) 💅

okay so this is the general commands manual page for flex and lex — the fast lexical analyzer generator. yes, both names are in the group chat. and no, we are not skipping a single crumb of this manual. periodt.

🌸 NAME 🌸

• flex, lex — the fast lexical analyzer generator. cute, concise, iconic.

🌈 SYNOPSIS 🌈

flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix -Sskeleton]
[--help --version] [filename ...]

🦋 OVERVIEW 🦋

bestie, this manual is giving both tutorial and reference energy. it literally says it includes both tutorial and reference sections, and here's the full lineup:

• Description — a brief overview of the tool.
• 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 your 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 parsers.
• Options — flex command-line options, and the "%option" directive.
• Performance Considerations — how to make your scanner 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 lex and the POSIX lex standard.
• Diagnostics — those error messages produced by flex (or scanners it generates) whose meanings might not be apparent.
• Files — files used by flex.
• Deficiencies / Bugs — known problems with flex.
• See Also — other documentation, related tools.
• Author — includes contact information.

💖 DESCRIPTION 💖

flex is a tool for generating scanners: programs which recognize lexical patterns in text. it 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 rules. flex generates as output a C source file, lex.yy.c, which defines a routine yylex(). this file is compiled and linked with the -ll 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. that's the whole main-character arc, fr fr.

🎀 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 pattern and the "printf" is the action. 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 increments 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 <math.h>
%}

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( argc, argv )
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 Pascal. it identifies different types of tokens and reports on what it has seen. the details of this example will be explained in the following sections.

🦋 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 definitions section contains declarations of simple name definitions to simplify the scanner specification, and declarations of start conditions, 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-white-space character following the name and continuing to the end of the line. the definition can subsequently be referred to using {name}, which will expand to (definition). for example,

DIGIT    [0-9]
ID       [a-z][a-z0-9]*

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 rules section of the flex input contains a series of rules of the form:

pattern   action

where 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 lex.yy.c verbatim. 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 unindented comment (i.e., a line beginning with "/*") is also copied verbatim to the output up to the next "*/".

🦋 PATTERNS 🦋

okay so the patterns in your input file are written with an extended set of regular expressions, and honestly the vibes are immaculate once you get them. Here's the whole roster, bestie — nothing skipped, promise:

• 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, an 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 Deficiencies / Bugs section below regarding "dangerous trailing context".)
• ^r — an r, but only at the beginning of a line (i.e., when 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.

heads up, bestie: flex's notion of "newline" is exactly whatever the C compiler used to compile flex interprets '\n' as; in particular, on some DOS systems you must either filter out \r's in the input yourself, or explicitly use r/\r\n for "r$". iykyk 💅

• <s>r — an r, but only in start condition s (see below for discussion of start conditions)
• <s1,s2,s3>r — same, but in any of start conditions s1, s2, or s3
• <*>r — an r in any start condition, even an exclusive one.
• <<EOF>> — an end-of-file
• <s1,s2><<EOF>> — an end-of-file when in start condition s1 or s2

Also, lowkey important: inside of a character class, all regular expression operators lose their special meaning except escape ('\') and the character class operators, '-', ']', and, at the beginning of the class, '^'. periodt.

---

✨ precedence, bestie ✨

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 example,

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 either the string foo or the string ba followed 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)*

slay, you got this 🌈

---

💜 character class expressions 💜

In addition to characters and ranges of characters, character classes can also contain character class expressions. These are expressions enclosed inside [: and :] delimiters (which themselves 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() returns true — i.e., any alphabetic or numeric. Some systems don't provide isblank(), 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 your 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. (yes this is a jump-scare, no I'm not sorry 😭)
• A rule can have at most one instance of trailing context (the '/' operator or the '$' operator). The start condition, '^', and "<<EOF>>" patterns can only occur at the beginning of a pattern, and, as well as with '/' and '$', cannot be grouped inside parentheses. A '^' which does not occur at the beginning 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$
<sc1>foo<sc2>bar

Note that the first of these, can be written "foo/bar\n".

The following will result in '$' or '^' being treated as a normal 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 🔥

So 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. longest match wins, ties go to whoever showed up first — main character rules only 💅

Once the match is determined, the text corresponding to the match (called the token) is made available in the global character pointer yytext, and its length in the global integer yyleng. The action corresponding to the matched pattern is then executed (a more detailed description of actions follows), and then the remaining 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. literally the "hello world" of flex, and it ate ✨

---

⭐ yytext: %pointer vs %array ⭐

Note that yytext can be defined in two different ways: either as a character pointer or as a character array. You can control which definition flex uses by including one of the special directives %pointer or %array in the first (definitions) section of your flex input. The default is %pointer, unless you use the -l lex compatibility option, in which case yytext will be an array.

The advantage of using %pointer is substantially faster scanning and no buffer overflow when matching very large tokens (unless you run out of dynamic memory). The disadvantage is that you are restricted in how your actions can modify yytext (see the next section), and calls to the unput() function destroys the present contents of yytext, which can be a considerable porting headache when moving between different lex versions. (ngl, the unput() thing is giving "surprise plot twist" 😭)

The advantage of %array is that you can then modify yytext to your heart's content, and calls to unput() do not destroy yytext (see below). Furthermore, existing lex programs sometimes access yytext externally using declarations of the form:

extern char yytext[];

This definition is erroneous when used with %pointer, but correct for %array.

%array defines yytext to be an array of YYLMAX characters, which defaults to a fairly large value. You can change the size by simply #define'ing YYLMAX to a different value in the first section of your flex input. As mentioned above, with %pointer yytext grows dynamically to accommodate large tokens. While this means your %pointer scanner can accommodate very large tokens (such as matching entire blocks of comments), bear in mind that each time the scanner must resize yytext it also must rescan the entire token from the beginning, so matching such tokens can prove slow. yytext 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 bestie, you cannot use %array with C++ scanner classes (the c++ option; see below). that's just the rules 🫶

💅 ACTIONS 💅

Okay so bestie, every pattern in a rule has a matching action, and that action can literally be any arbitrary C statement you want. No cap. The pattern stops at the first non-escaped whitespace character, and everything after that on the line? That's your action. And if the action is just… empty? Then when the pattern matches, the input token is simply yeeted into the void. Gone. Discarded.

Like, here's a whole program that deletes every occurrence of "zap me" from its input:

%%
"zap me"

(And it'll copy all the other characters in the input to the output, because those get matched by the default rule. slay.)

Here's one that squishes multiple blanks and tabs down to a single blank, and throws away whitespace at the end of a line:

%%
[ \t]+        putchar( ' ' );
[ \t]+$       /* ignore this token */

Now, if an action contains a {, then the action spans until the balancing } is found — and it can totally cross multiple lines, main character energy. flex knows about C strings and comments so it won't get fooled by braces hiding inside them, but it also lets actions begin with %{, in which case it considers the action to be all the text up to the next %} (regardless of ordinary braces inside the action). iykyk.

An action that's solely a vertical bar (|) means "same as the action for the next rule." There's a lil illustration below, don't worry 🫶

Actions can include arbitrary C code, including return statements that return a value to whatever routine called yylex(). Every time yylex() gets called, it keeps processing tokens from where it last left off until it either hits the end of the file or executes a return. Periodt.

Actions are free to modify yytext except for lengthening it (adding characters to its end — those will overwrite later characters in the input stream). This does not apply when you're using %array (see above); in that case, yytext may be freely modified in any way. 💖

Actions are also free to modify yyleng — except they should not do so if the action also includes use of yymore() (see below).

---

There are a number of special directives you can drop inside an action:

• ECHO — copies yytext to the scanner's output. cute and simple.
• BEGIN followed by the name of a start condition — places the scanner 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 "How the Input is Matched", and yytext and yyleng set up appropriately. It may either be one which matched as much text as the originally 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 REJECT, any "frob"'s in the input would not be counted as words, since the scanner normally executes only one action per token. Multiple REJECT'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. ✨)

REJECT 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, REJECT cannot be used with the -Cf or -CF options (see below).

Also note that unlike the other special actions, REJECT is a branch; 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 yytext 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 yytext, so the ECHO for the "kludge" rule will actually write "mega-kludge". iconic behavior honestly.

Two notes regarding use of yymore(). First, yymore() depends on the value of yyleng correctly reflecting the size of the current token, so you must not modify yyleng if you are 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 n characters of the current token back to the input stream, where they will be rescanned when the scanner looks for the next match. yytext and yyleng are adjusted appropriately (e.g., yyleng will now be equal to n ). For example, on the input "foobar" the following will write out "foobarbar":

%%
foobar    ECHO; yyless(3);
[a-z]+    ECHO;

An argument of 0 to yyless will cause the entire current input string to be scanned again. Unless you've changed how the scanner will subsequently process its input (using BEGIN, for example), this will result in an endless loop. 🙈

Note that yyless is a macro and can only be used in the flex input file, not from other source files.

• unput(c) — puts the character c back onto 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;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
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(): if you are using %pointer (the default), a call to unput() destroys the contents of yytext, starting with its rightmost character and devouring one character to the left with each call. If you need the value of yytext preserved after a call to unput() (as in the above example), you must either first copy it elsewhere, or build your scanner using %array instead (see How The Input Is Matched).

Finally, note that you cannot put back EOF to attempt to mark the input stream with an end-of-file. it's just not that girl.

• input() — reads the next character from the input stream. For example, 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 )
                    {
                    error( "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 The Generated Scanner, below). This action is a special case of the more general yy_flush_buffer() function, described below in the section Multiple Input Buffers.
• 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 ✨

Okay bestie, so flex spits out a file called lex.yy.c, and inside it lives the whole scanner situation. You get the scanning routine yylex(), a bunch of tables it uses to match tokens, plus a whole squad of auxiliary routines and macros. By default, yylex() is declared like this:

int yylex()
    {
    ... various definitions and the actions in here ...
    }

(And if your environment supports function prototypes, then it's gonna be "int yylex( void )". Either way, slay.) You can change this whole definition by defining the "YY_DECL" macro. For example, you could use:

#define YY_DECL float lexscan( a, b ) float a, b;

...to give your scanning routine the name lexscan, returning a float, and taking two floats as arguments. Iconic, honestly. 💅 Just note that if you give arguments to the scanning routine using a K&R-style / non-prototyped function declaration, you must terminate the definition with a semi-colon (;). Don't forget it, that's the whole vibe check.

---

Every single time yylex() is called, it scans tokens from the global input file yyin (which defaults to stdin). It keeps going until it either hits an end-of-file — at which point it returns the value 0 — or one of its actions executes a return statement.

Heads up bestie: if the scanner reaches an end-of-file, subsequent calls are undefined unless either yyin is pointed at a new input file (in which case scanning continues from that file), or yyrestart() is called. 👀

yyrestart() takes one argument, a FILE * pointer (which can be nil, if you've set up YY_INPUT to scan from a source other than yyin), and initializes yyin for scanning from that file. Essentially there is no difference between just assigning yyin 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 yyin — but better is to use YY_FLUSH_BUFFER (see above). And note that yyrestart() does not reset the start condition to INITIAL (see Start Conditions, below).

If yylex() stops scanning because one of the actions executed a return statement, the scanner may then be called again and it will resume scanning right where it left off. No lost progress, we love that for us. 🫶

---

By default (and for purposes of efficiency), the scanner uses block-reads rather than simple getc() calls to read characters from yyin. 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 buf and return in the integer variable result 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 character at a time. Tiny sips of input, adorable. 🍓

---

When the scanner receives an end-of-file indication from YY_INPUT, it then checks the yywrap() function. Here's the tea ☕ — if yywrap() returns false (zero), then it is assumed that the function has gone ahead and set up yyin to point to another input file, and scanning continues. But 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 INITIAL.

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 -ll to obtain the default version of the routine, which always returns 1. Pick your fighter, bestie. ✨

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 Multiple Input Buffers.

The scanner writes its ECHO output to the yyout global (default: stdout), which may be redefined by the user simply by assigning it to some other FILE pointer. Redirect your echo, live your truth. 🌈

✨ START CONDITIONS ✨

okay so this is the part where flex gets actually unhinged in the best way 💅 flex has a whole mechanism for conditionally activating rules — like, rules that only wake up when the scanner is in a certain mood.

Any rule whose pattern is prefixed with <sc> will only be active when the scanner is in the start condition named sc. for example:

<STRING>[^"]*        { /* eat up the string body ... */
            ...
            }

this one is only alive when the scanner is in the STRING start condition. and then:

<INITIAL,STRING,QUOTE>\.        { /* handle an escape ... */
            ...
            }

this one is active when the current start condition is either INITIAL, STRING, or QUOTE. multiple vibes, one rule 🫶

---

Start conditions get declared in the definitions (first) section of the input, using unindented lines that begin with either %s or %x, followed by a list of names. the first one — %s — declares inclusive start conditions, and %x declares exclusive start conditions. yes bestie, exclusive. it's giving main character energy 👑

A start condition gets activated with the BEGIN action. Until the next BEGIN action runs, rules with that given start condition are active and rules with other start conditions are inactive. Simple.

now here's the tea on inclusive vs exclusive:

• if the start condition is inclusive, then rules with no start conditions at all are also active. everybody's invited to the party 🎀
• if it's exclusive, then only rules qualified with that start condition are active. strict guest list, no plus-ones 💀

A set of rules contingent on the same exclusive start condition describes a scanner that is independent of any of the other rules in the flex input. because of this, exclusive start conditions make it super easy to specify "mini-scanners" that scan portions of the input which are syntactically different from the rest (e.g., comments). little sub-scanners, lowkey iconic 🌸

---

if the distinction between inclusive and exclusive is still a little vague, here's a simple example illustrating the connection between the two. the set of rules:

%s example
%%

<example>foo   do_something();

bar            something_else();

is equivalent to:

%x example
%%

<example>foo   do_something();

<INITIAL,example>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 INITIAL — 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. see how that works? the way that %s just lets everything in 😭

Also note that the special start-condition specifier <*> matches every start condition. so the above example could also have been written as:

%x example
%%

<example>foo   do_something();

<*>bar    something_else();

The default rule (to ECHO any unmatched character) remains active in start conditions. it's 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 INITIAL, so BEGIN(INITIAL) is equivalent to BEGIN(0). (the parentheses around the start condition name are not required but are considered good style. be the kind of person who uses them 💅)

---

BEGIN 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 enter_special is true:

            int enter_special;

%x SPECIAL
%%
        if ( enter_special )
            BEGIN(SPECIAL);

<SPECIAL>blahblahblah
...more rules follow...

---

To illustrate the uses of start conditions, here's 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 <math.h>
%}
%s expect

%%
expect-floats        BEGIN(expect);

<expect>[0-9]+"."[0-9]+      {
            printf( "found a float, = %f\n",
                    atof( yytext ) );
            }
<expect>\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's 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);

<comment>[^*\n]*        /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */
<comment>\n             ++line_num;
<comment>"*"+"/"        BEGIN(INITIAL);

This scanner goes to a bit of trouble to match as much text as possible with each rule. In general, when attempting to write a high-speed scanner, try to match as much possible in each rule, as it's a big win. fr fr, greedy rules = fast scanner 🔥

---

Note that start-conditions names are really integer values and can be stored as such. thus, the above could be extended in the following fashion:

%x comment foo
%%
        int line_num = 1;
        int comment_caller;

"/*"         {
             comment_caller = INITIAL;
             BEGIN(comment);
             }

...

<foo>"/*"    {
             comment_caller = foo;
             BEGIN(comment);
             }

<comment>[^*\n]*        /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */
<comment>\n             ++line_num;
<comment>"*"+"/"        BEGIN(comment_caller);

Furthermore, you can access the current start condition using the integer-valued YY_START macro. for example, the above assignments to comment_caller could instead be written:

comment_caller = YY_START;

Flex provides YYSTATE as an alias for YY_START (since that's what's used by AT&T lex). legacy support, we love to see it 🫶

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. just out here sharing the namespace, no boundaries, no cap 🙈

---

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

%%
        char string_buf[MAX_STR_CONST];
        char *string_buf_ptr;

\"      string_buf_ptr = string_buf; BEGIN(str);

<str>\"        { /* saw closing quote - all done */
        BEGIN(INITIAL);
        *string_buf_ptr = '\0';
        /* return string constant token type and
         * value to parser
         */
        }

<str>\n        {
        /* error - unterminated string constant */
        /* generate error message */
        }

<str>\\[0-7]{1,3} {
        /* octal escape sequence */
        int result;

        (void) sscanf( yytext + 1, "%o", &result );

        if ( result > 0xff )
                /* error, constant is out-of-bounds */

        *string_buf_ptr++ = result;
        }

<str>\\[0-9]+ {
        /* generate error - bad escape sequence; something
         * like '\48' or '\0777777'
         */
        }

<str>\\n  *string_buf_ptr++ = '\n';
<str>\\t  *string_buf_ptr++ = '\t';
<str>\\r  *string_buf_ptr++ = '\r';
<str>\\b  *string_buf_ptr++ = '\b';
<str>\\f  *string_buf_ptr++ = '\f';

<str>\\(.|\n)  *string_buf_ptr++ = yytext[1];

<str>[^\\\n\"]+        {
        char *yptr = yytext;

        while ( *yptr )
                *string_buf_ptr++ = *yptr++;
        }

---

Often, such as in some of the examples above, you wind up writing a whole bunch of rules all preceded by the same start condition(s). Flex makes this a little easier and cleaner by introducing a notion of start condition scope. A start condition scope is begun with:

<SCs>{

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 like, group them together bestie, it's giving organization 🌈

So, for example:

<ESC>{
    "\\n"   return '\n';
    "\\r"   return '\r';
    "\\f"   return '\f';
    "\\0"   return '\0';
}

is equivalent to:

<ESC>"\\n"  return '\n';
<ESC>"\\r"  return '\r';
<ESC>"\\f"  return '\f';
<ESC>"\\0"  return '\0';

Start condition scopes may be nested. yes, nested. inception vibes 🌀

---

Three routines are available for manipulating stacks of start conditions:

• void yy_push_state(int new_state) — pushes the current start condition onto the top of the start condition stack and switches to new_state as though you had used BEGIN new_state (recall that start condition names are also integers).
• void yy_pop_state() — pops the top of the stack and switches to it via BEGIN.
• int yy_top_state() — returns the top of the stack without altering the stack's contents. peek, don't touch 👀

The start condition stack grows dynamically and so has no built-in size limitation. If memory is exhausted, program execution aborts. like, straight up aborts 💀 no graceful exit, just done.

To use start condition stacks, your scanner must include a %option stack directive (see Options below). don't forget it bestie, the stack doesn't exist without it 🎀

💖 MULTIPLE INPUT BUFFERS 💖

Okay bestie, so picture this: some scanners (the ones that do the whole "include files" thing, iykyk) need to read from several input streams at once. And here's the catch — flex scanners buffer a lot, so you literally cannot control where the next input comes from just by writing a YY_INPUT that's all sensitive to the scanning context. Like, YY_INPUT only gets called when the scanner hits the end of its buffer, and that could be waaaay after it scanned a statement like an "include" that needs the input source switched. 😭 Not the vibe.

So to deal with this whole situation, flex gives you a whole mechanism for creating and switching between multiple input buffers. A buffer gets created with:

YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

which takes a FILE pointer and a size and makes a buffer tied to that file, big enough to hold size characters (when in doubt, just use YY_BUF_SIZE for the size, bestie 🫶). It hands you back a YY_BUFFER_STATE handle, which you can then pass to the other routines (more on those below). The YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state structure, so you can safely initialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE) 0) if that's what you want, and you can also refer to the opaque structure to correctly declare input buffers in source files other than the one with your scanner in it. 🎀

Also super important: the FILE pointer you pass to yy_create_buffer is only used as the value of yyin seen by YY_INPUT. So if you redefine YY_INPUT so it doesn't use yyin anymore, then you can pass a nil FILE pointer to yy_create_buffer and it's totally fine. ✨

You pick which buffer to scan from with:

void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

which switches the scanner's input buffer so all the subsequent tokens come out of new_buffer. Ngl, yy_switch_to_buffer() can also be used by yywrap() to set things up for continued scanning instead of opening a new file and pointing yyin at it. 👑 And note note note: switching input sources through either yy_switch_to_buffer() or yywrap() does not change the start condition. Periodt.

void yy_delete_buffer( YY_BUFFER_STATE buffer )

is the one you use to reclaim the storage that a buffer was using. (buffer can be nil, and in that case the routine just does nothing. Iconic, honestly.) You can also wipe the current contents of a buffer with:

void yy_flush_buffer( YY_BUFFER_STATE buffer )

This function literally discards the buffer's contents, so the next time the scanner tries to match a token from that buffer, it'll fill the buffer anew using YY_INPUT first. 🦋

yy_new_buffer() is just an alias for yy_create_buffer(), provided for compatibility with the C++ use of new and delete for creating and destroying dynamic objects. Slay.

And finally, the YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle to the current buffer. 💅

Here's an example using all these features to write a scanner that expands include files (the <<EOF>> feature gets discussed further down, don't worry):

/* 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;

<incl>[ \t]*      /* eat the whitespace */
<incl>[^ \t\n]+   { /* got the include file name */
        if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
            {
            fprintf( stderr, "Includes nested too deeply" );
            exit( 1 );
            }

        include_stack[include_stack_ptr++] =
            YY_CURRENT_BUFFER;

        yyin = fopen( yytext, "r" );

        if ( ! yyin )
            error( ... );

        yy_switch_to_buffer(
            yy_create_buffer( yyin, YY_BUF_SIZE ) );

        BEGIN(INITIAL);
        }

<<EOF>> {
        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 so you can scan in-memory strings instead of files. All three of them create a new input buffer for scanning the string and return the matching YY_BUFFER_STATE handle (which you should delete with yy_delete_buffer() when you're done with it, bestie — clean up after yourself 🧚). They also switch to the new buffer using yy_switch_to_buffer(), so the very next call to yylex() starts scanning the string. 🌸

• yy_scan_string(const char *str) — scans a NUL-terminated string.
• yy_scan_bytes(const char *bytes, int len) — scans len bytes (including possibly NUL's) starting at location bytes.

Just so you know, both of these functions create and scan a copy of the string or bytes. (This may be exactly what you want, since yylex() modifies the contents of the buffer it's scanning. 🙈) You can dodge the copy entirely by using:

• yy_scan_buffer(char *base, yy_size_t size) — scans in place the buffer starting at base, consisting of size bytes, where the last two bytes must be YY_END_OF_BUFFER_CHAR (ASCII NUL). Those last two bytes are not scanned; so scanning consists of base[0] through base[size-2], inclusive.

If you fail to set up base in this manner (i.e., you 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. 💀 So like, don't forget them.

The type yy_size_t is an integral type to which you can cast an integer expression reflecting the size of the buffer.

---

🌈 END-OF-FILE RULES 🌈

The special rule <<EOF>> indicates the actions to take when an end-of-file is encountered and yywrap() returns non-zero (i.e., it's telling you there are no further files to process). The action has to finish by doing one of these four things:

• assigning yyin to a new input file (in previous versions of flex, after doing the assignment you had to call the special action YY_NEW_FILE; this is no longer necessary 🎉);
• executing a return statement;
• executing the special yyterminate() action;
• or, 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 you give an unqualified <<EOF>> rule, it applies to all start conditions that don't already have <<EOF>> actions. To specify an <<EOF>> rule for only the initial start condition, use:

<INITIAL><<EOF>>

These rules are super useful for catching things like unclosed comments. Example time:

%x quote
%%

...other rules for dealing with quotes...

<quote><<EOF>>   {
         error( "unterminated quote" );
         yyterminate();
         }
<<EOF>>  {
         if ( *++filelist )
             yyin = fopen( *filelist, "r" );
         else
            yyterminate();
         }

---

✨ MISCELLANEOUS MACROS ✨

The macro YY_USER_ACTION can be defined to give you an action that's always executed right before the matched rule's action. For example, it could be #define'd to call a routine that converts yytext to lower-case. When YY_USER_ACTION is invoked, the variable yy_act gives you the number of the matched rule (rules are numbered starting with 1, cute). Say you wanna profile how often each of your rules gets matched — the following would do the trick:

#define YY_USER_ACTION ++ctr[yy_act]

where ctr is an array holding 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 you use -s), so a correct declaration for ctr is:

int ctr[YY_NUM_RULES];

The macro YY_USER_INIT may be defined to provide an action that's always executed before the first scan (and before the scanner's internal initializations are done). Like, 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 interactive. An interactive buffer is processed more slowly, but it must be used when the scanner's input source really is interactive, to avoid problems from 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 using this macro overrides %option interactive, %option always-interactive or %option never-interactive (see Options below). And yy_set_interactive() must be invoked before you begin scanning 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 it were 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, and false otherwise. 💜

In the generated scanner, all the actions are gathered into one big switch statement and separated using YY_BREAK, which may be redefined. By default it's simply a break, separating each rule's action from the following rule's. Redefining YY_BREAK lets, for example, C++ users #define YY_BREAK to do nothing (while being very careful that every rule ends with a break or a return! 😭) so they can avoid suffering from unreachable statement warnings — because when a rule's action ends with return, the YY_BREAK is inaccessible. 🔥

✨ VALUES AVAILABLE TO THE USER ✨

Okay bestie, this section is basically the goodie bag 💅 — everything that's handed to you inside a rule action. Here's the full rundown:

• char *yytext holds the text of the current token. You can modify it, but you absolutely cannot lengthen it — no appending characters to the end, periodt. 🚫

If the special directive %array shows up in the first section of the scanner description, then yytext is instead declared as char yytext[YYLMAX], where YYLMAX is a macro definition you can redefine in the first section if the default value isn't your vibe (generally 8KB). Using %array gives you somewhat slower scanners, but the value of yytext becomes immune to calls to input() and unput(), which can potentially destroy its value when yytext is a character pointer. The opposite of %array is %pointer, which is the default.

Heads up: you cannot use %array when generating C++ scanner classes (the -+ flag). Not allowed, no cap. 🙈

• int yyleng holds the length of the current token.
• FILE *yyin is the file which by default flex reads from. You can redefine it, 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, you can assign yyin at the new input file and then call the scanner again to continue scanning. 🎀
• void yyrestart( FILE *new_file ) may be called to point yyin at the new input file. The switch-over to the new file is immediate (any previously buffered-up input is lost — rip 😭). Note that calling yyrestart() with yyin as an argument thus throws away the current input buffer and continues scanning the same input file.
• FILE *yyout is the file to which ECHO 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. You can subsequently use this value with BEGIN to return to that start condition. Iconic little round-trip, love that for you. ✨

---

🌈 INTERFACING WITH YACC 🌈

One of the main uses of flex is as a companion to the yacc parser-generator — they're literally a duo, main character energy fr fr 👑. yacc parsers expect to call a routine named yylex() to find the next input token. That routine is supposed to return the type of the next token as well as putting any associated value in the global yylval.

To use flex with yacc, you specify the -d option to yacc to instruct it to generate the file y.tab.h containing definitions of all the %tokens appearing in the yacc input. That file then gets included in the flex scanner. For example, if one of the tokens is "TOK_NUMBER", part of the scanner might look like this:

%{
#include "y.tab.h"
%}

%%

[0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;

Slay. 💅

---

💖 OPTIONS 💖

flex has the following options — and yes, we're going through all of them, no skipping, we stay thorough in this house:

• -b, --backup — Generate backing-up information to lex.backup. 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 Performance Considerations below.)
• -c — is a do-nothing, deprecated option included for POSIX compliance. It does literally nothing, it's just there for the vibes 🫶.
• -d, --debug — makes the generated scanner run in debug mode. Whenever a pattern is recognized and the global yy_flex_debug 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, --full — specifies fast scanner. No table compression is done and stdio is bypassed. The result is large but fast. This option is equivalent to -Cfr (see below). Big scanner, big speed, big slay 🔥.
• -h, --help — generates a "help" summary of flex's options to stdout and then exits. -? and --help are synonyms for -h.
• -i, --case-insensitive — 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 yytext will have the preserved case (i.e., it will not be folded). ✨
• -l, --lex-compat — turns on maximum compatibility with the original AT&T lex implementation. Note that this does not mean full compatibility. 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 "Incompatibilities With Lex And POSIX" below. This option also results in the name YY_FLEX_LEX_COMPAT being #define'd in the generated scanner.
• -n — is another do-nothing, deprecated option included only for POSIX compliance. Also doing nothing, also iconic in its uselessness 💀.
• -p, --perf-report — generates a performance report to stderr. The report consists of comments regarding features of the flex input file which will cause a serious loss of performance in the resulting scanner. If you give the flag twice, you will also get comments regarding features that lead to minor performance losses.

Note that the use of REJECT, %option yylineno, and variable trailing context (see the Deficiencies / Bugs section below) entails a substantial performance penalty; use of yymore(), the ^ operator, and the -I flag entail minor performance penalties.

• -s, --no-default — causes the default rule (that unmatched scanner input is echoed to stdout) to be suppressed. If the scanner encounters 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, --stdout — instructs flex to write the scanner it generates to standard output instead of lex.yy.c.
• -v, --verbose — 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, --nowarn — suppresses warning messages. Ignorance is bliss, bestie 🙈.
• -B, --batch — instructs flex to generate a batch scanner, the opposite of interactive scanners generated by -I (see below). In general, you use -B when you are certain that your scanner will never be used interactively, and you want to squeeze a little more performance out of it. If your goal is instead to squeeze out a lot more performance, you should be using the -Cf or -CF options (discussed below), which turn on -B automatically anyway.
• -F, --fast — 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 you're better off using the full table representation. If only the "identifier" rule is present and you then use a hash table or some such to detect the keywords, you're better off using -F.

This option is equivalent to -CFr (see below). It cannot be used with -+.

• -I, --interactive — instructs flex to generate an interactive 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 disambiguate the current token, is a bit faster than only looking 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 newline token until they enter another token, which often means typing in another whole line. 😭

Flex scanners default to interactive unless you use the -Cf or -CF table-compression options (see below). That's because if you're looking for high-performance you should be using one of these options, so if you didn't, flex assumes you'd rather trade off a bit of run-time performance for intuitive interactive behavior. Note also that you cannot use -I 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.

Note that if isatty() returns false for the scanner input, flex will revert to batch mode, even if -I was specified. To force interactive mode no matter what, use %option always-interactive (see Options below).

You can force a scanner to not be interactive by using -B (see above).

• -L, --noline — 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 correctly located with respect to either the original flex input file (if the errors are due to code in the input file), or lex.yy.c (if the errors are flex's fault — you should report these sorts of errors to the email address given below). 🎀
• -T, --trace — makes flex run in trace mode, bestie. It will generate a lot of messages to stderr concerning the form of the input and the resultant non-deterministic and deterministic finite automata. This option is mostly for use in maintaining flex. 👀
• -V, --version — prints the version number to stdout and exits. --version is a synonym for -V. 💅
• -7, --7bit — tells 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. 😭

Okay but here's the nuance: unless you generate your scanner using the -Cf or -CF table compression options, use of -7 will save only a small amount of table space, and make your scanner considerably less portable. Flex's default behavior is to generate an 8-bit scanner unless you use -Cf or -CF, in which case flex defaults to generating 7-bit scanners unless your site was always configured to generate 8-bit scanners (as will often be the case with non-USA sites). You can tell whether flex generated a 7-bit or an 8-bit scanner by inspecting the flag summary in the -v output as described above. 🕵️

And note that if you use -Cfe or -CFe (those 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, --8bit — tells 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 otherwise 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. ✨
• -+, --c++ — specifies that you want flex to generate a C++ scanner class. See the section on Generating C++ Scanners below for details. 🎀
• -C[aefFmr] — controls the degree of table compression and, more generally, trade-offs between small scanners and fast scanners. This is the pick your fighter of table vibes. ⚖️
  • -Ca, --align — (the docs literally call it "align") 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 architectures, 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 your scanner. 🔥
  • -Ce, --ecs — directs flex to construct equivalence classes, i.e., sets of characters which have identical lexical properties. 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 reductions 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 full scanner tables should be generated — flex should not compress the tables by taking advantage of similar transition functions for different states. 🧚
  • -CF — specifies that the alternative fast scanner representation (described above under the -F flag) should be used. This option cannot be used with -+. 🚫
  • -Cm, --meta-ecs — directs flex to construct meta-equivalence classes, which are sets of equivalence classes (or characters, 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, --read — causes the generated scanner to bypass use of the standard I/O library (stdio) for input. Instead of calling fread() or getc(), the scanner will use the read() system call, resulting in a performance gain which varies from system to system, but in general is probably negligible unless you are also using -Cf or -CF. Using -Cr can cause strange behavior if, for example, you read from yyin using stdio prior to calling the scanner (because the scanner will miss whatever text your previous reads left in the stdio input buffer). 😬

-Cr has no effect if you define YY_INPUT (see The Generated Scanner above). 🫶

A lone -C specifies that the scanner tables should be compressed but neither equivalence classes nor meta-equivalence 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. You can 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 you will usually want to use the default, maximal compression. 💅

-Cfe is often a good compromise between speed and size for production scanners. 👑

• -ooutput, --outputfile=FILE — directs flex to write the scanner to the file output instead of lex.yy.c. If you combine -o with the -t option, then the scanner is written to stdout but its #line directives (see the -L option above) refer to the file output. 📝
• -Pprefix, --prefix=STRING — changes the default yy prefix used by flex for all globally-visible variable and function names to instead be prefix. For example, -Pfoo changes the name of yytext to footext. It also changes the name of the default output file from lex.yy.c to lex.foo.c. 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 you are using a C++ scanner, then only yywrap and yyFlexLexer are affected.) Within your scanner itself, you can still refer to the global variables and functions using either version of their name; but externally, they have the modified name. 🦋

This option lets you easily link together multiple flex programs into the same executable. Note, though, that using this option also renames yywrap(), so you now must either provide your own (appropriately-named) version of the routine for your scanner, or use %option noyywrap, as linking with -ll no longer provides one for you by default. 🔗

• -Sskeleton_file, --skel=FILE — overrides the default skeleton file from which flex constructs its scanners. You'll never need this option unless you are doing flex maintenance or development. 🛠️
• -X, --posix-compat — maximal compatibility with POSIX lex. 🧾
• --yylineno — track line count in yylineno. 📈
• --yyclass=NAME — name of C++ class. 🎓
• --header-file=FILE — create a C header file in addition to the scanner. 📁
• --tables-file[=FILE] — write tables to FILE. 🗂️
• -Dmacro[=defn] — #define macro defn (default defn is '1'). 🧬
• -R, --reentrant — generate a reentrant C scanner. 🔁
• --bison-bridge — scanner for bison pure parser. 🐃
• --bison-locations — include yylloc support. 📍
• --stdinit — initialize yyin/yyout to stdin/stdout. 🏁
• --noansi-definitions — old-style function definitions. 🕰️
• --noansi-prototypes — empty parameter list in prototypes. 📜
• --nounistd — do not include <unistd.h>. 🚫
• --noFUNCTION — do not generate a particular FUNCTION. 🙅

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. You can specify multiple options with a single %option directive, and multiple directives in the first section of your 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 — the -7 option
• 8bit — the -8 option
• align — the -Ca option
• backup — the -b option
• batch — the -B option
• c++ — the -+ option
• caseful or case-sensitive — opposite of -i (default)
• case-insensitive or caseless — the -i option
• debug — the -d option
• default — opposite of -s option
• ecs — the -Ce option
• fast — the -F option
• full — the -f option
• interactive — the -I option
• lex-compat — the -l option
• meta-ecs — the -Cm option
• perf-report — the -p option
• read — the -Cr option
• stdout — the -t option
• verbose — the -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 — tells flex to make a scanner that always treats its input as "interactive". Normally, on every new input file, the scanner calls isatty() to try to figure out whether its input source is interactive and therefore should be read one character at a time. With this option, tho? No such call is made. 💅
• main — tells flex to hand you a default main() program for the scanner, and that program literally just calls yylex(). This option implies noyywrap (peep below 👀).
• never-interactive — tells flex to make a scanner that never treats its input as "interactive" (again, no call to isatty() is made). It’s the opposite of always-interactive. ✨
• stack — enables the use of start condition stacks (see Start Conditions above 🫶).
• stdinit — if set (i.e., %option stdinit), initializes yyin and yyout 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 — tells flex to generate a scanner that maintains the number of the current line read from its input in the global variable yylineno. 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 yyin at a new file and calls yylex() again). 💀

---

flex scans your rule actions to determine whether you use the REJECT or yymore() features. The reject and yymore options are available to override its decision as to whether you use the options — either by setting them (e.g., %option reject) to indicate 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 -oABC, and
• %option prefix="XYZ" is equivalent to -PXYZ. Finally,
• %option yyclass="foo" only applies when generating a C++ scanner (-+ option). It informs flex that you have derived foo as a subclass of yyFlexLexer, so flex will place your 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 Generating C++ Scanners, below, for additional information. 🧚

---

A number of options are available for lint purists who want to suppress 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 scanner:

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 you use %option stack). 🙈

⚡ PERFORMANCE CONSIDERATIONS ⚡

Okay so here's the thing, bestie — flex's whole main-character design goal is generating high-performance scanners, and it's literally been optimized to deal well with large sets of rules. ✨ But some options/actions do degrade performance, and here they are, ranked 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()

The first three? Quite expensive. The last two? Quite cheap. Also note that unput() is implemented as a routine call that potentially does quite a bit of work, while yyless() is a quite-cheap macro — so if you're just putting back some excess text you scanned, use yyless(). Simple as that. 💅

REJECT should be avoided at all costs when performance is important. It's a particularly expensive option. Like, no cap, don't do it.

Getting rid of backing up is messy and often may be an enormous amount of work for a complicated scanner. In principle, one begins by using the -b flag to generate a lex.backup 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 compressed 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). Careful with that one, bestie.

It's important to keep in mind that you gain the benefits of eliminating backing up only if you eliminate every instance of backing up. Leaving just one means you gain nothing. Literally zero. 🙈

Variable trailing context (where both the leading and trailing parts do not have a fixed length) entails almost the same performance loss as REJECT (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 Deficiencies / Bugs 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. Why? 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., yytext) for the action. Recall the scanner for C comments:

%x comment
%%
        int line_num = 1;

"/*"         BEGIN(comment);

<comment>[^*\n]*
<comment>"*"+[^*/\n]*
<comment>\n             ++line_num;
<comment>"*"+"/"        BEGIN(INITIAL);

This could be sped up by writing it as:

%x comment
%%
        int line_num = 1;

"/*"         BEGIN(comment);

<comment>[^*\n]*
<comment>[^*\n]*\n      ++line_num;
<comment>"*"+[^*/\n]*
<comment>"*"+[^*/\n]*\n ++line_num;
<comment>"*"+"/"        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. And get this — adding rules does not slow down the scanner! The speed of the scanner is independent of the number of rules or (modulo the considerations 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: suppose you want to scan through a file containing 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 How the Input is Matched, dynamically resizing yytext 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, you should 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 ✨

Okay so flex has two whole ways to make C++ scanners, and bestie, both are valid — here’s the tea 💅

Way #1: just compile the flex-generated scanner with a C++ compiler instead of a C compiler. That’s literally it. You should not encounter any compilation errors (please report any you find to the email address given in the Author section below). You can then use C++ code in your rule actions instead of C code. Note that the default input source for your scanner remains yyin, and default echoing is still done to yyout. Both of these remain FILE * variables and not C++ streams. No cap.

---

Way #2: use flex to generate a C++ scanner class with the -+ option (or, equivalently, %option c++), which is automatically specified if the name of the flex executable ends in a '+', such as flex++. When you use this option, flex defaults to generating the scanner to the file lex.yy.cc instead of lex.yy.c. The generated scanner includes the header file FlexLexer.h, which defines the interface to two C++ classes.

The first class, FlexLexer, provides an abstract base class defining 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 yytext.
• int YYLeng() — returns the length of the most recently matched token, the equivalent of yyleng.
• 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 yy_flex_debug (see the Options section above). Note that you must build the scanner 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 argument is an std::istream* object pointer and not a FILE*), yy_flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is an std::istream* object pointer).

The second class defined in FlexLexer.h is yyFlexLexer, which is derived from FlexLexer. It defines the following additional member functions:

• yyFlexLexer( std::istream* arg_yyin = 0, std::ostream* arg_yyout = 0 ) — constructs a yyFlexLexer object using the given streams for input and output. If not specified, the streams default to cin and cout, respectively.
• virtual int yylex() — performs the same role yylex() does for ordinary flex scanners: it scans the input stream, consuming tokens, until a rule’s action returns a value. If you derive a subclass S from yyFlexLexer and want to access the member functions and variables of S inside yylex(), then you need to use %option yyclass="S" to inform flex that you will be using that subclass instead of yyFlexLexer. 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 yyin to new_in (if non-nil) and yyout to new_out (ditto), deleting the previous input buffer if yyin 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, yyFlexLexer defines the following protected virtual functions which you can redefine in derived classes to tailor the scanner:

• virtual int LexerInput( char* buf, int max_size ) — reads up to max_size characters into buf 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 you redefine LexerInput() and need to take different actions depending on whether or not the scanner might be scanning an interactive input source, you can test for the presence of this name via #ifdef.
• virtual void LexerOutput( const char* buf, int size ) — writes out size characters from the buffer buf, 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 cerr and exits.

Note that a yyFlexLexer object contains its entire scanning state. Thus you can use such objects to create reentrant scanners. You can instantiate multiple instances of the same yyFlexLexer class, and you can also combine multiple C++ scanner classes together in the same program using the -P option discussed above.

Finally, note that the %array feature is not available to C++ scanner classes; you must use %pointer (the default).

Here is an example of a simple C++ scanner:

               // An example of using the flex C++ scanner class.

           %{
           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;
               }

If you want to create multiple (different) lexer classes, you use the -P flag (or the prefix= option) to rename each yyFlexLexer to some other xxFlexLexer. You then can include <FlexLexer.h> in your other sources once per lexer class, first renaming yyFlexLexer as follows:

#undef yyFlexLexer
#define yyFlexLexer xxFlexLexer
#include <FlexLexer.h>

#undef yyFlexLexer
#define yyFlexLexer zzFlexLexer
#include <FlexLexer.h>

if, for example, you used %option prefix="xx" for one of your scanners and %option prefix="zz" 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 💅

ok so bestie, flex is a full-on rewrite of the AT&T Unix lex tool (like, they share zero code, wild), and it comes with some extensions and some incompatibilities — and both of those matter if you're trying to write a scanner that works with either implementation. Here's the tea: flex is fully compliant with the POSIX lex specification, except for one lil thing — when you're using %pointer (which is the default), a call to unput() absolutely nukes the contents of yytext, and that's counter to the POSIX spec. 😭

In this section we go through literally all the known areas of incompatibility between flex, AT&T lex, and the POSIX specification.

flex's -l option turns on maximum compatibility with the original AT&T lex implementation, at the cost of a major loss in the generated scanner's performance. We note below which incompatibilities can be overcome using the -l option.

flex is fully compatible with lex with the following exceptions:

• The undocumented lex scanner internal variable yylineno is not supported unless -l or %option yylineno is used.

yylineno should be maintained on a per-buffer basis, rather than a per-scanner (single global variable) basis.

yylineno 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 yyin.

• The unput() routine is not redefinable. This restriction is in accordance with POSIX. 👍
• flex scanners are not as reentrant as lex scanners. In particular, if you have an interactive scanner and an interrupt handler which long-jumps out of the scanner, and the scanner is subsequently called again, you may get the following message:

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 for you, you should use them instead. See "Generating C++ Scanners" above for details. ✨

• output() is not supported. Output from the ECHO macro is done to the file-pointer yyout (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. flex said "I'm gonna do it anyway" 💅
• When definitions are expanded, flex encloses them in parentheses. 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. slay.

Note that if the definition begins with ^ or ends with $ then it is not expanded with parentheses, to allow these operators 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<space here>
  { foobar_action(); }

flex does not support this feature. sorry bestie 🙈

• The lex %r (generate a Ratfor scanner) option is not supported. It is not part of the POSIX specification.
• After a call to unput(), yytext 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 different. 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 interprets "^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()
• <<EOF>>
• <*>
• 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 you can put multiple actions on the same line, separated with semi-colons, while with lex, the following

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.

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🔥 DIAGNOSTICS 🔥

• warning, rule cannot be matched — indicates that the given rule cannot 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 REJECT 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 or yymore_used_but_not_detected undefined — These errors can occur at compile time. They indicate that the scanner uses REJECT 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 you snuck some in (via a #include file, for example). Use %option reject or %option yymore to indicate to flex that you really do use these features.
• 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 — your scanner uses %array and one of its rules matched a string longer than the YYLMAX constant (8K bytes by default). You can increase the value by #define'ing YYLMAX in the definitions section of your flex input.
• scanner requires -8 flag to use the character 'x' — Your scanner specification includes recognizing the 8-bit character 'x' and you did not specify the -8 flag, and your scanner defaulted to 7-bit because you used the -Cf or -CF table compression options. See the discussion of the -7 flag for details.
• flex scanner push-back overflow — you used unput() to push back so much text that the scanner's buffer could not hold both the pushed-back text and the current token in yytext. 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 REJECT.
• fatal flex scanner internal error--end of buffer missed — This can occur in a 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! — you listed more start conditions in a <> construct than exist (so you must have listed at least one of them twice). 🙈

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🎀 FILES 🎀

• -ll — library with which scanners must be linked.
• lex.yy.c — generated scanner (called lexyy.c on some systems).
• lex.yy.cc — generated C++ scanner class, when using -+.
• <FlexLexer.h> — header file defining the C++ scanner base class, FlexLexer, and its derived class, yyFlexLexer.
• flex.skl — skeleton scanner. This file is only used when building flex, not when flex executes.
• lex.backup — backing-up information for -b flag (called lex.bck on some systems).

💀 DEFICIENCIES / BUGS 💀

Okay bestie, gather round, because flex is iconic but she is not perfect, and we need to talk about it. 💅

• Some trailing context patterns cannot be properly matched and generate 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, like "zx*/xy*", where the 'x*' matches the 'x' at the beginning of the trailing context. Important: the POSIX draft states that the text matched by such patterns is undefined. So like… don't be surprised if it acts up. 😭
• For some trailing context rules, parts that are actually fixed-length are not recognized as such, which leads to that performance loss we just mentioned. In particular, parts using '|' or {n} (such as "foo{3}") are always considered variable-length. It's giving "close enough" energy, but make it slower. 🫠
• 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 this cutie right here:

%%
abc      |
xyz/def

• Use of unput() invalidates yytext and yyleng, unless the %array directive or the -l option has been used. So yeah, keep that in mind before you go unputting things. 🙈
• Pattern-matching of NUL's is substantially slower than matching other characters. Not a vibe, but it is what it is. 💀
• Dynamic resizing of the input buffer is slow, because it entails rescanning all the text matched so far by the current (generally huge) token. Big token energy = big rescan energy. 😭
• Due to both buffering of input and read-ahead, you cannot intermix calls to <stdio.h> routines — such as, for example, getchar() — with flex rules and expect it to work. Call input() instead. This is one of those "please actually listen" moments. 🫶
• 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 REJECT, and somewhat greater than the number of states if it does. Math girlies rise. 📊✨
• REJECT cannot be used with the -f or -F options. Periodt. 🚫
• The flex internal algorithms need documentation. Not me calling out the docs in the docs… 💀

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🔗 SEE ALSO 🔗

Go read your sisters and cousins, bestie:

• lex(1)
• yacc(1)
• sed(1)
• awk(1)

Also, the required reading list is giving iconic:

• John Levine, Tony Mason, and Doug Brown, Lex & Yacc, O'Reilly and Associates. Be sure to get the 2nd edition. This is not a drill. 📚
• M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator
• Alfred Aho, Ravi Sethi and Jeffrey Ullman, Compilers: Principles, Techniques and Tools, Addison-Wesley (1986). This one describes the pattern-matching techniques used by flex (deterministic finite automata). DFA girlies, we love you. 🦋

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👑 AUTHOR 👑

Okay the author credits are a whole parade and we are not skipping a single person, because everyone contributed and we respect the squad. 💖

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 Paxson.

Thanks to the many flex beta-testers, feedbackers, and contributors, especially:

Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Adermann, Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, Nelson H.F. Beebe, benson@odi.com, Karl Berry, Peter A. Bigot, Simon Blanchard, Keith Bostic, Frederic Brehm, Ian Brockbank, 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 Hofmann, 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, ken@ken.hilco.com, Kevin B. Kenny, Steve Kirsch, Winfried 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 Martensson, 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 Richardson, 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. If you're in this list, know that you ate. 💅

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 distribution headaches. Distribution headaches are so real, and we appreciate you. 😭🫶

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 buffers. Legends, all of them. ✨

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 vern@ee.lbl.gov. Yes, that's the actual address in the page, bestie — copy it exactly. 📬

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And that's the whole flex(1) page, babes. This was Version 2.6.4, dated May 6, 2017, and you just finished FLEX(1) — go forth and scan something iconic. 👑💖✨