FLEX(1)
💖 FLEX(1) — bestie edition 💖
General Commands Manual · FLEX(1)
✨ NAME ✨
flex — the fast lexical analyzer generator, aka the tool that eats your regexes and spits out a whole scanner 🧚
🌸 SYNOPSIS 🌸
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix -Sskeleton]
[--help --version] [filename ...]🦋 OVERVIEW 🦋
Okay so this manual is your guide to flex, the tool for generating programs that do pattern-matching on text. It's got a whole tutorial half and a reference half, because it's generous like that. Here's the map:
- Description — a brief overview of the tool
- Some Simple Examples — exactly what it says, lil starter snacks
- Format Of The Input File — how to structure your flex 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
flexproduces; 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
flexscanners together withyaccparsers - Options —
flexcommand-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
flexdiffers from AT&Tlexand the POSIXlexstandard - 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 recognized lexical patterns in text. So it reads the input files you hand it — or its standard input if you don't give it any file names — and that input is a description of a scanner it's gonna generate. That description comes as pairs of regular expressions and C code, and those pairs are called rules. 💅
Then flex generates a C source file as output, lex.yy.c, which defines a routine called yylex(). You compile that and link it with the -lfl library, and boom — you've got an executable. Run that executable and it analyzes its input looking for occurrences of those regular expressions. Every time it finds one, it runs the matching C code. Literally the whole vibe. ✨
🎀 SOME SIMPLE EXAMPLES 🎀
Okay bestie, first some simple examples so you can feel how flex gets used. 🫶
This next flex input describes a scanner that, whenever it bumps into the string "username", swaps it out for the user's login name:
%%
username printf( "%s", getlogin() );By default, any text that a flex scanner doesn't match just gets copied to the output, so the net effect of this scanner is: copy the input file to the output, except every "username" gets expanded. In this input there's exactly one rule. "username" is the pattern and the "printf" is the action. And the "%%" marks the beginning of the rules. slay.
Here's another simple one:
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, so it's the strong silent type). 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 matching a newline ("\n") that bumps both the line count and the character count, and one matching any character that isn't a newline (that's the "." regular expression) which bumps just the char count. 🌈
And now 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. (No skipping ahead, bestie, we're doing this properly.)
🧚 FORMAT OF THE INPUT FILE 🧚
A flex input file has three sections, separated by a line that's just %% in it:
definitions
%%
rules
%%
user codeThe definitions section holds declarations of simple name definitions (to make your scanner spec less of a slog) and declarations of start conditions, which get explained in a later section.
Name definitions look like this:
name definitionThe "name" is a word that starts 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 after the name and continues all the way to the end of the line. Later on you can refer to it with "{name}", which expands to "(definition)". For example:
DIGIT [0-9]
ID [a-z][a-z0-9]*defines "DIGIT" to be a regular expression matching a single digit, and "ID" to be a regular expression matching a letter followed by zero-or-more letters-or-digits. So a later reference to
{DIGIT}+"."{DIGIT}*is identical to
([0-9])+"."([0-9])*and it matches one-or-more digits followed by a '.' followed by zero-or-more digits. 💅
The rules section of the flex input holds a series of rules shaped like:
pattern actionwhere the pattern has to be unindented and the action has to begin on the same line. (Yes, the whitespace is doing real work here, she's not just decorative.)
See below for a further description of patterns and actions.
Finally, the user code section is simply copied to lex.yy.c verbatim. It's for companion routines that call the scanner or get called by it. This section is optional — if it's 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 %} gets 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 that are local to the scanning routine, and (after the declarations) code that's to be executed whenever the scanning routine is entered. Other indented or %{} text in the rules section still gets 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 — that's a line beginning with "/*" — is also copied verbatim to the output up to the next "*/".
okay we're just getting started — more parts incoming, don't go anywhere 💖
🌈 PATTERNS 🌈
okay bestie, this is the part where flex gets spicy ✨ — the patterns in your input are written using an extended set of regular expressions, and here's the whole menu:
x match the character 'x'
. any character (byte) except newline
[xyz] a "character class"; in this case, the pattern
matches either an 'x', a 'y', or a 'z'
[abj-oZ] a "character class" with a range in it; matches
an 'a', a 'b', any letter from 'j' through 'o',
or a 'Z'
[^A-Z] a "negated character class", i.e., any character
but those in the class. In this case, any
character EXCEPT an uppercase letter.
[^A-Z\n] any character EXCEPT an uppercase letter or
a newline
r* zero or more r's, where r is any regular expression
r+ one or more r's
r? zero or one r's (that is, "an optional r")
r{2,5} anywhere from two to five r's
r{2,} two or more r's
r{4} exactly 4 r's
{name} the expansion of the "name" definition
(see above)
"[xyz]\"foo"
the literal string: [xyz]"foo
\X if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
then the ANSI-C interpretation of \x.
Otherwise, a literal 'X' (used to escape
operators such as '*')
\0 a NUL character (ASCII code 0)
\123 the character with octal value 123
\x2a the character with hexadecimal value 2a
(r) match an r; parentheses are used to override
precedence (see below)
rs the regular expression r followed by the
regular expression s; called "concatenation"
r|s either an r or an s
r/s an r but only if it is followed by an s. The
text matched by s is included when determining
whether this rule is the "longest match",
but is then returned to the input before
the action is executed. So the action only
sees the text matched by r. This type
of pattern is called trailing context".
(There are some combinations of r/s that flex
cannot match correctly; see notes in the
Deficiencies / Bugs section below regarding
"dangerous trailing context".)
^r an r, but only at the beginning of a line (i.e.,
which just starting to scan, or right after a
newline has been scanned).
r$ an r, but only at the end of a line (i.e., just
before a newline). Equivalent to "r/\n".
Note that flex's notion of "newline" is exactly
whatever the C compiler used to compile flex
interprets '\n' as; 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$".
<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 s2quick heads up: once you're inside a character class, all those regex operators lose their special meaning — <em>except</em> for the escape '\', the character class operators, '-', ']', and, if it shows up at the very beginning of the class, '^'. 💅
✨ the precedence tea ✨
The regular expressions above are grouped by precedence, highest at the top, lowest at the bottom. Anything grouped together has equal precedence. So for example:
foo|bar*is literally the same as
(foo)|(ba(r*))because the '*' operator outranks concatenation, and concatenation outranks alternation ('|'). So that pattern matches either the string "foo" or the string "ba" followed by zero-or-more r's. If you actually wanted "foo" or zero-or-more "bar"'s, you'd write:
foo|(bar)*and if you want zero-or-more "foo"'s-or-"bar"'s, you'd write:
(foo|bar)*💖 character class expressions 💖
okay so character classes don't just hold characters and ranges — they can also hold character class expressions. These are expressions wrapped in [: and :] delimiters (and those delimiters have to sit between the '[' and ']' of the character class; other stuff can hang out in there too, no worries). The valid ones are:
[:alnum:] [:alpha:] [:blank:]
[:cntrl:] [:digit:] [:graph:]
[:lower:] [:print:] [:punct:]
[:space:] [:upper:] [:xdigit:]Each of these designates a set of characters equivalent to the matching standard C isXXX function. Like, [:alnum:] gives you every character where isalnum() returns true — so any alphabetic or numeric character. And heads up: some systems don't provide isblank(), so flex defines [:blank:] as a blank or a tab. 🫶
So for example, all of these character classes are equivalent:
[[:alnum:]]
[[:alpha:][:digit:]
[[:alpha:]0-9]
[a-zA-Z0-9]And 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 like the
"[^A-Z]"example up there 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 <em>unlike</em> how a lot of other regular expression tools treat negated character classes, but unfortunately the inconsistency is historically entrenched. 😭 And matching newlines means a pattern like[^"]*can match the entire input unless there's another quote in the input. 💀 - A rule can have at most one instance of trailing context (the
'/'operator or the'$'operator). The start condition,'^', and"<<EOF>>"patterns can only occur at the beginning of a pattern, and, just like with'/'and'$', they cannot be grouped inside parentheses. A'^'that doesn't occur at the beginning of a rule, or a'$'that doesn't occur at the end of a rule, loses its special properties and gets treated as a normal character.
The following are illegal:
foo/bar$
<sc1>foo<sc2>barNote 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|^barIf what's wanted is a "foo" or a bar-followed-by-a-newline, the following could be used (the special '|' action is explained below):
foo |
bar$ /* action goes here */A similar trick will work for matching a foo or a bar-at-the-beginning-of-a-line. 👑
🦋 HOW THE INPUT IS MATCHED 🦋
When the generated scanner runs, it analyzes its input looking for strings that match any of its patterns. If it finds more than one match, it takes the one matching the most text (and for trailing context rules, that includes the length of the trailing part, even though it then gets returned to the input). If it finds two or more matches of the <em>same</em> length, the rule listed first in the flex input file wins. periodt. 💅
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. Then the action corresponding to the matched pattern is executed (there's a more detailed description of actions coming up), and then the remaining input is scanned for another match.
If no match is found, the default rule runs: the next character in the input is considered matched and copied to the standard output. Which means the simplest legal flex input is:
%%...which generates a scanner that simply copies its input (one character at a time) to its output. iconic. 🌈
💜 yytext: pointer or array? the eternal question 💜
Note that yytext can be defined in two different ways: either as a character pointer or as a character array. You control which one 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're restricted in how your actions can modify yytext (see the next section), and calls to the unput() function destroy the present contents of yytext, which can be a considerable porting headache when moving between different lex versions. 😭
The advantage of %array is that you can then modify yytext to your heart's content, and calls to unput() do <em>not</em> destroy yytext (see below). On top of that, 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 that means your %pointer scanner can handle very large tokens (like matching entire blocks of comments 🫶), bear in mind that every time the scanner has to resize yytext it also has to rescan the entire token from the beginning, so matching those tokens can prove slow. And 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 note that you cannot use %array with C++ scanner classes (the c++ option; see below).
💥 ACTIONS 💥
okay bestie so every pattern in a rule has a matching action, and that action can literally be any arbitrary C statement you want. no rules, no limits, go feral. ✨
here's the tea on where the pattern ends: the pattern stops at the first non-escaped whitespace character, and everything after that whitespace on the line is the action. simple as that.
and if the action is empty? then when that pattern matches, the input token just gets yeeted into the void. discarded. gone. no cap.
for example, here's a whole spec for a program that deletes every occurrence of "zap me" from its input:
%%
"zap me"(it will copy all the other characters in the input to the output, because the default rule catches them — she's doing her job 💅)
and here's a lil program that compresses multiple blanks and tabs down to a single blank, and throws away whitespace found at the end of a line:
%%
[ \t]+ putchar( ' ' );
[ \t]+$ /* ignore this token */🌀 the curly-brace situation
if your action contains a '{', then the action spans all the way till the balancing '}' shows up, and it can cross multiple lines, no problem. flex knows about C strings and comments so she won't get fooled by braces hiding inside them, periodt. 🫶
she also lets actions begin with %{, in which case the action is all the text up to the next %} — regardless of any ordinary braces inside the action.
the '|' shortcut
an action consisting solely of a vertical bar ('|') means "same as the action for the next rule." see below for a whole illustration, it's giving efficiency. 💖
what actions can actually do
actions can include arbitrary C code, including return statements to return a value to whatever routine called yylex(). every time yylex() is called, it keeps processing tokens from where it last left off, until it either reaches the end of the file or executes a return. main character energy fr. 👑
- 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 using%array(see above); in that caseyytextmay be freely modified in any way. - actions are free to modify
yyleng— except you should not do that if the action also includes use ofyymore()(see below).
🎀 special directives inside an action 🎀
ECHO— copiesyytextto the scanner's output.BEGINfollowed 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", andyytextandyylengare 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.) 🦋
⚠️REJECTis 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,REJECTcannot be used with the-Cfor-CFoptions (see below). 💀
also note: 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 ofyytextrather 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". obsessed with this behavior honestly.
two notes regarding use of yymore():
yymore()depends on the value ofyylengcorrectly reflecting the size of the current token, so you must not modifyyylengif you are usingyymore().- 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.yytextandyylengare adjusted appropriately (e.g.,yylengwill 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 of0toyylesswill cause the entire current input string to be scanned again. unless you've changed how the scanner will subsequently process its input (usingBEGIN, for example), this will result in an endless loop. be so serious about this one.
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. iykyk. 💅
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. nope, not happening bestie. 🙈
input()— reads the next character from the input stream. for example, the following is one way to eat up C comments:
%%
"/*" {
register 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 usingYY_INPUT(see The Generated Scanner, below). this action is a special case of the more generalyy_flush_buffer()function, described below in the section Multiple Input Buffers. 🧚yyterminate()— can be used in lieu of areturnstatement in an action. it terminates the scanner and returns a0to 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 one file and that file is lex.yy.c 💖 and inside it lives the actual scanning routine yylex(), plus a bunch of tables it uses to match tokens, plus a whole little friend group of auxiliary routines and macros. by default, yylex() is declared like this:
int yylex()
{
... various definitions and the actions in here ...
}(if your environment supports function prototypes, then it's int yylex( void ) instead — same vibe, just fancier ✨)
and the way that you can change this definition? you define the YY_DECL macro. like, for example, you could do:
#define YY_DECL float lexscan( a, b ) float a, b;and boom — now your scanning routine is named lexscan, it returns a float, and it takes two floats as arguments. main character rebrand 💅
⚠️ heads up though: 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 (;). no cap, don't forget it.
Every single time yylex() gets called, it scans tokens from the global input file yyin (which defaults to stdin, as one does 🍓). 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.
Now, if the scanner reaches an end-of-file, subsequent calls are undefined 😭 unless either:
yyinis pointed at a new input file (in which case scanning continues from that file), oryyrestart()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 it initializes yyin for scanning from that file. Lowkey there's essentially no difference between just assigning yyin to a new input file and using yyrestart() to do it — the latter exists 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). 💀
also note:yyrestart()does not reset the start condition toINITIAL(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 exactly where it left off. it's giving "we were interrupted, but we're so back" 🦋
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)
and 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.
Here's a sample definition of YY_INPUT (living 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 only, bestie 🧚
So when the scanner receives an end-of-file indication from YY_INPUT, it then checks the yywrap() function. and here's the tea:
- If
yywrap()returns false (zero), then it's assumed that the function has gone ahead and set upyyinto point to another input file, and scanning continues. 🔥 - If it returns true (non-zero), then the scanner terminates, returning
0to its caller.
📌 Note that in either case, the start condition remains unchanged — it does not revert to INITIAL.
And if you don't supply your own version of yywrap(), then you have exactly two options, bestie, pick one:
- use
%option noyywrap(in which case the scanner behaves as thoughyywrap()returned1), or - link with
-lflto obtain the default version of the routine, which always returns1.
Three routines are available for scanning from in-memory buffers rather than files, and they are iconic:
yy_scan_string()yy_scan_bytes()yy_scan_buffer()
see the discussion of them below in the section Multiple Input Buffers 👑
Last but never least: 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. customize ur output stream, it's literally ur right 🌸
✨ START CONDITIONS ✨
okay so flex has this whole mechanism for conditionally activating rules, and it's literally the bestie of state machines 💖. 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 is active only when the scanner is in the "STRING" start condition, and:
<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}...will be active only when the current start condition is either "INITIAL", "STRING", or "QUOTE". periodt.
Start conditions are declared in the definitions (first) section of the input using unindented lines beginning with either %s or %x followed by a list of names. The former declares inclusive start conditions, the latter exclusive start conditions. A start condition is activated using the BEGIN action. Until the next BEGIN action is executed, rules with the given start condition will be active and rules with other start conditions will be inactive. If the start condition is inclusive, then rules with no start conditions at all will also be active. If it is exclusive, then only rules qualified with the start condition will be active. A set of rules contingent on the same exclusive start condition describe a scanner which is independent of any of the other rules in the flex input. Because of this, exclusive start conditions make it easy to specify "mini-scanners" which scan portions of the input that are syntactically different from the rest (e.g., comments). slay.
If the distinction between inclusive and exclusive start conditions is still a little vague, here's a simple example illustrating the connection between the two. The set of rules:
%s example
%%
<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 startion condition is an inclusive (%s) start condition.
Also note that the special start-condition specifier <*> matches every start condition. Thus, the above example could also have been written;
%x example
%%
<example>foo do_something();
<*>bar something_else();The default rule (to ECHO any unmatched character) remains active in start conditions. It is equivalent to:
<*>.|\n ECHO;BEGIN(0) returns to the original state where only the rules with no start conditions are active. This state can also be referred to as the start-condition "INITIAL", so BEGIN(INITIAL) is equivalent to BEGIN(0). (The parentheses around the start condition name are not required but are considered good style.) good style check ✅
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 is a scanner which provides two different interpretations of a string like "123.456". By default it will treat it as three tokens, the integer "123", a dot ('.'), and the integer "456". But if the string is preceded earlier in the line by the string "expect-floats" it will treat it as a single token, the floating-point number 123.456:
%{
#include <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 is a scanner which recognizes (and discards) C comments while maintaining a count of the current input line.
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<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. big win energy 💅
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 is what's used by AT&T lex).
Note that start conditions do not have their own name-space; %s's and %x's declare names in the same fashion as #define's. 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, 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. nesting? we love a nested moment 🪆
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 tonew_stateas though you had usedBEGIN new_state(recall that start condition names are also integers).void yy_pop_state()— pops the top of the stack and switches to it viaBEGIN.int yy_top_state()— returns the top of the stack without altering the stack's contents.
The start condition stack grows dynamically and so has no built-in size limitation. If memory is exhausted, program execution aborts. bestie, memory exhaustion is NOT the vibe 😭
To use start condition stacks, your scanner must include a %option stack directive (see Options below).
🫶 MULTIPLE INPUT BUFFERS 🫶
Okay so some scanners (think the ones that support "include" files, yk) need to read from several input streams at once. And here's the thing — flex scanners do a lot of buffering, so you literally cannot control where the next input comes from just by writing a YY_INPUT that's sensitive to the scanning context. Like, YY_INPUT only gets called when the scanner hits the end of its buffer, which could be forever after you scanned a statement like an "include" that needs to switch the input source. It's giving "I'll get to it when I get to it" 😭
So to negotiate all that, flex gives you a whole mechanism for creating and switching between multiple input buffers. You make an input buffer like this:
YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )It takes a FILE pointer and a size, and creates a buffer tied to that file that's big enough to hold size characters (when in doubt, use YY_BUF_SIZE for the size). It hands you back a YY_BUFFER_STATE handle that you can pass to the other routines (see below, bestie 💅).
The YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state — so yes, you can safely initialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE) 0) if that's your vibe, and you can also refer to the opaque structure so you can correctly declare input buffers in source files that aren't your scanner's. Slay.
Heads up: theFILEpointer in the call toyy_create_bufferis only used as the value ofyyinseen byYY_INPUT. So if you redefineYY_INPUTso it no longer usesyyin, then you can safely pass a nilFILEpointer toyy_create_buffer. No cap.
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 from new_buffer. Note that yy_switch_to_buffer() can be used by yywrap() to set things up for continued scanning, instead of opening a new file and pointing yyin at it. Also note: switching input sources via either yy_switch_to_buffer() or yywrap() does not change the start condition. Periodt.
void yy_delete_buffer( YY_BUFFER_STATE buffer )is what you use to reclaim the storage associated with a buffer. (buffer can be nil, in which case the routine does absolutely nothing — iconic, no drama.) You can also clear the current contents of a buffer using:
void yy_flush_buffer( YY_BUFFER_STATE buffer )This function discards the buffer's contents, so the next time the scanner tries to match a token from the buffer, it'll first fill the buffer anew using YY_INPUT. Fresh start energy ✨
yy_new_buffer() is an alias for yy_create_buffer(), provided for compatibility with the C++ use of new and delete for creating and destroying dynamic objects. 🎀
And finally, the YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle to the current buffer. 💖
Here's an example of using these features to write a scanner that expands include files (the <<EOF>> feature is discussed below, don't worry bestie):
/* 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] );
}
}There are three routines for setting up input buffers to scan in-memory strings instead of files. All of them create a new input buffer for scanning the string, and return a corresponding YY_BUFFER_STATE handle (which you should delete with yy_delete_buffer() when you're done with it, obviously 🫶). They also switch to the new buffer using yy_switch_to_buffer(), so the next call to yylex() will start scanning the string. Chef's kiss.
yy_scan_string(const char *str)— scans a NUL-terminated string.yy_scan_bytes(const char *bytes, int len)— scanslenbytes (including possibly NUL's) starting at locationbytes.
Note that both of these functions create and scan a copy of the string or bytes. (This may be desirable, since yylex() modifies the contents of the buffer it's scanning — it's a little gremlin like that 🙈.) You can avoid the copy by using:
yy_scan_buffer(char *base, yy_size_t size)— which scans in place the buffer starting atbase, consisting ofsizebytes, the last two bytes of which must beYY_END_OF_BUFFER_CHAR(ASCII NUL). These last two bytes are not scanned; thus, scanning consists ofbase[0]throughbase[size-2], inclusive.
If you fail to set up base in this manner (i.e., forget the final two YY_END_OF_BUFFER_CHAR bytes), then yy_scan_buffer() returns a nil pointer instead of creating a new input buffer. So like... don't do that 💀
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 actions which are to be taken when an end-of-file is encountered and yywrap() returns non-zero (i.e., it indicates there are no further files to process). The action has to finish by doing one of four things, bestie, pick your fighter:
- assigning
yyinto a new input file (in previous versions of flex, after doing the assignment you had to call the special actionYY_NEW_FILE; this is no longer necessary — glow up ✨); - executing a
returnstatement; - 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 an unqualified <<EOF>> rule is given, it applies to all start conditions which do not already have <<EOF>> actions. To specify an <<EOF>> rule for only the initial start condition, use
<INITIAL><<EOF>>These rules are useful for catching things like unclosed comments. An example:
%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 provide an action which is always executed prior to the matched rule's action. For example, it could be #define'd to call a routine to convert yytext to lower-case. When YY_USER_ACTION is invoked, the variable yy_act gives the number of the matched rule (rules are numbered starting with 1). Suppose you want to profile how often each of your rules is matched — the following would do the trick:
#define YY_USER_ACTION ++ctr[yy_act]where ctr is an array to hold the counts for the different rules. Note that the macro YY_NUM_RULES gives the total number of rules (including the default rule, even if 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 which is always executed before the first scan (and before the scanner's internal initializations are done). For example, it could be used to call a routine to read in a data table or open a logging file. Very main character of it. 👑
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 must be used when the scanner's input source is indeed interactive, to avoid problems due to waiting to fill buffers (see the discussion of the -I flag below). A non-zero value in the macro invocation marks the buffer as interactive, a zero value as non-interactive. Note that use of this macro overrides %option always-interactive or %option never-interactive (see Options below). yy_set_interactive() must be invoked prior to beginning to scan the buffer that is (or is not) to be considered interactive. Okay so timing matters here, bestie, don't skip that step.
The macro yy_set_bol(at_bol) can be used to control whether the current buffer's scanning context for the next token match is done as though at the beginning of a line. A non-zero macro argument makes rules anchored with
The macro YY_AT_BOL() returns true if the next token scanned from the current buffer will have '^' rules active, false otherwise.
In the generated scanner, the actions are all gathered in one large switch statement and separated using YY_BREAK, which may be redefined. By default, it is simply a "break", to separate each rule's action from the following rule's. Redefining YY_BREAK allows, for example, C++ users to #define YY_BREAK to do nothing (while being very careful that every rule ends with a "break" or a "return"!) to avoid suffering from unreachable statement warnings where, because a rule's action ends with "return", the YY_BREAK is inaccessible. 😭
✨ VALUES AVAILABLE TO THE USER ✨
okay bestie, this is the lil treasure chest of stuff you can grab inside your rule actions 🎀 here's the whole rundown:
char *yytextholds the text of the current token. you can modify it, but you cannot make it longer — nope, no appending characters to the end, that's illegal bestie 💀
if the special directive %array shows up in the first section of the scanner description, then yytext gets declared as char yytext[YYLMAX] instead, where YYLMAX is a macro definition you're free to redefine in the first section if the default value is not your vibe (it's 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%arraywhen generating C++ scanner classes (the-+flag). periodt.
int yylengholds the length of the current token 💅FILE *yyinis the file which by default flex reads from. you can redefine it, but honestly doing so only makes sense before scanning begins or after an EOF has been encountered. changing it in the middle of scanning will have unexpected results since flex buffers its input — useyyrestart()instead, bestie 🙈 once scanning terminates because an end-of-file has been seen, you can assignyyinat the new input file and then call the scanner again to continue scanning ✨void yyrestart( FILE *new_file )may be called to pointyyinat the new input file. the switch-over to the new file is immediate (any previously buffered-up input is lost 😭). note that callingyyrestart()withyyinas an argument thus throws away the current input buffer and continues scanning the same input file.FILE *yyoutis the file to whichECHOactions are done. it can be reassigned by the user 🫶YY_CURRENT_BUFFERreturns aYY_BUFFER_STATEhandle to the current buffer 💖YY_STARTreturns an integer value corresponding to the current start condition. you can subsequently use this value withBEGINto return to that start condition 👑
🌈 INTERFACING WITH YACC 🌈
one of the main uses of flex is as a companion to the yacc parser-generator, and honestly? iconic duo energy 👑✨
yacc parsers expect to call a routine named yylex() to find the next input token. the routine is supposed to return the type of the next token as well as putting any associated value in the global yylval. to use flex with yacc, one specifies 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. this file is then included in the flex scanner. for example, if one of the tokens is "TOK_NUMBER", part of the scanner might look like:
%{
#include "y.tab.h"
%}
%%
[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;✨ OPTIONS ✨
flex has the following options, and bestie we are going through them, all of them, no skips 💅
-b— generate backing-up information tolex.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-Cfor-CFis used, the generated scanner will run faster (see the-pflag). 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 🥱-d— makes the generated scanner run in debug mode. whenever a pattern is recognized and the globalyy_flex_debugis non-zero (which is the default), the scanner will write tostderra line of the form:
--accepting rule at line 53 ("the matched text")the line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to flex). messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; at this point, the two look the same as far as the scanner's concerned), or reaches an end-of-file 🔍
-f— specifies 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) 🏎️-h— generates a "help" summary of flex's options tostdoutand then exits.-?and--helpare synonyms for-h🫶-i— instructs flex to generate a case-insensitive scanner. the case of letters given in the flex input patterns will be ignored, and tokens in the input will be matched regardless of case. the matched text given inyytextwill have the preserved case (i.e., it will not be folded) ✨-l— 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-CFoptions. for details on the compatibilities it provides, see the section "Incompatibilities With Lex And POSIX" below. this option also results in the nameYY_FLEX_LEX_COMPATbeing#define'd in the generated scanner 💅-n— is another do-nothing, deprecated option included only for POSIX compliance 🥱-p— generates a performance report tostderr. 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— causes the default rule (that unmatched scanner input is echoed tostdout) 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— instructs flex to write the scanner it generates to standard output instead oflex.yy.c📝-v— specifies that flex should write tostderra summary of statistics regarding the scanner it generates. most of the statistics are meaningless to the casual flex user, but the first line identifies the version of flex (same as reported by-V), and the next line the flags used when generating the scanner, including those that are on by default 📊-w— suppresses warning messages 🤐-B— instructs flex to generate a batch scanner, the opposite of interactive scanners generated by-I(see below). in general, you use-Bwhen 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-Cfor-CFoptions (discussed below), which turn on-Bautomatically anyway 🏃-F— specifies that the fast scanner table representation should be used (and stdio bypassed). this representation is about as fast as the full table representation (-f), and for some sets of patterns will be considerably smaller (and for others, larger). in general, if the pattern set contains both "keywords" and a catch-all, "identifier" rule, such as in the set:
"case" return TOK_CASE;
"switch" return TOK_SWITCH;
...
"default" return TOK_DEFAULT;
[a-z]+ return TOK_ID;then 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— 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 🫶
you can force a scanner to not be interactive by using -B (see above).
-L— instructs flex not to generate#linedirectives. without this option, flex peppers the generated scanner with#linedirectives 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), orlex.yy.c(if the errors are flex's fault — you should report these sorts of errors to the email address given below) 📮-T— makes flex run in trace mode. it will generate a lot of messages tostderrconcerning the form of the input and the resultant non-deterministic and deterministic finite automata. this option is mostly for use in maintaining flex 🔬-V— prints the version number tostdoutand exits.--versionis a synonym for-V👑-7— tells flex to make a 7-bit scanner, meaning it can only recognize 7-bit characters in its input. The glow-up? Your tables can be up to half the size of the ones you'd get with-8. The catch? These scanners will very often just hang or crash the moment an 8-bit character shows up in their input. 💀
But bestie, listen — unless you're generating your scanner with the -Cf or -CF table compression options, using -7 saves you like, a tiny amount of table space and makes your scanner considerably less portable. Flex's default behavior is to generate an 8-bit scanner unless you use -Cf or -CF, and in that case flex defaults to 7-bit scanners unless your site was always configured to generate 8-bit scanners (which is often the case with non-USA sites). You can tell whether flex made you a 7-bit or 8-bit scanner by peeking at the flag summary in the -v output as described above. 👀
Also note: if you use -Cfe or -CFe (those compression options plus equivalence classes), flex still defaults to an 8-bit scanner, because with those compression options full 8-bit tables aren't really much more expensive than 7-bit tables. No cap.
-8— tells flex to generate an 8-bit scanner, i.e. one that can recognize 8-bit characters. You only actually need this flag for scanners generated with-Cfor-CF, because otherwise flex defaults to an 8-bit scanner anyway. ✨
See the -7 discussion above for flex's default behavior and the whole 7-bit vs 8-bit tradeoff saga.
-+— says you want flex to generate a C++ scanner class. See the section on Generating C++ Scanners below for the details, bestie. 🎀-C[aefFmr]— controls how much table compression happens and, more broadly, the trade-offs between small scanners and fast scanners. It's giving very customizable. 💅-Ca("align") — tells flex to trade off larger tables in the generated scanner for faster performance, because the table elements end up better aligned for memory access and computation. On some RISC architectures, fetching and manipulating longwords is more efficient than smaller-sized units like shortwords. Heads up: this option can double the size of the tables your scanner uses. 😭-Ce— directs flex to construct equivalence classes, i.e. sets of characters that have identical lexical properties. (Like, if the only place digits show up in your flex input is the character class"[0-9]", then'0','1', ...,'9'all get put in the same equivalence class.) Equivalence classes usually give dramatic reductions in final table/object file sizes (typically a factor of 2-5) and they're pretty cheap performance-wise — just one array look-up per character scanned. Iconic behavior honestly. 👑-Cf— says the full scanner tables should be generated; flex shouldn't compress the tables by taking advantage of similar transition functions for different states.-CF— says to use the alternate fast scanner representation (described above under the-Fflag). This one cannot be used with-+. 🚫-Cm— directs flex to construct meta-equivalence classes, which are sets of equivalence classes (or characters, if you're not using equivalence classes) that are commonly used together. Meta-equivalence classes are often a big win with compressed tables, but they have a moderate performance impact — one or two"if"tests and one array look-up per character scanned.-Cr— makes the generated scanner bypass the standard I/O library (stdio) for input. Instead of callingfread()orgetc(), the scanner will use theread()system call. That gives you a performance gain that varies from system to system, but is probably negligible in general unless you're also using-Cfor-CF. And watch out — using-Crcan cause strange behavior if, for example, you read fromyyinusing stdio before calling the scanner, because the scanner will miss whatever text your previous reads left sitting in the stdio input buffer. 🙈-Crhas no effect if you defineYY_INPUT(see The Generated Scanner above).
A lone -C says 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's no opportunity for meta-equivalence classes if the table isn't being compressed. Otherwise the options may be freely mixed, and they're cumulative.
The default setting is -Cem, which says flex should generate equivalence classes and meta-equivalence classes. This setting gives you the highest degree of table compression. You can trade off faster-executing scanners at the cost of larger tables, and the following is generally true:
slowest & smallest
-Cem
-Cm
-Ce
-C
-C{f,F}e
-C{f,F}
-C{f,F}a
fastest & largestNote that scanners with the smallest tables are usually generated and compiled the quickest, so while you're developing you'll usually want the default, maximal compression. 🐛
-Cfe is often a good compromise between speed and size for production scanners. 🫶
-ooutput— directs flex to write the scanner to the file output instead oflex.yy.c. And if you combine-owith the-toption, then the scanner is written to stdout, but its#linedirectives (see the-Loption above) refer to the file output.-Pprefix— changes the defaultyyprefix that flex uses for all globally-visible variable and function names, and makes it prefix instead. So for example,-Pfoochanges the name ofyytexttofootext. It also changes the name of the default output file fromlex.yy.ctolex.foo.c. Here's every name that gets affected, bestie:
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're 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 multiple flex programs together into the same executable. Note though: 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, since linking with -lfl no longer provides one for you by default. 😤
-Sskeleton_file— overrides the default skeleton file from which flex constructs its scanners. You will never need this option unless you're doing flex maintenance or development. Like, never ever. It's giving insider-only. 🤫
flex also gives you a way to control options from inside the scanner specification itself, rather than from the flex command line. You do this 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 of them are equivalent to flex flags or their negation:
7bit— the-7option8bit— the-8optionalign— the-Caoptionbackup— the-boptionbatch— the-Boptionc++— the-+optioncasefulorcase-sensitive— the opposite of-i(this is the default)case-insensitiveorcaseless— the-ioptiondebug— the-doptiondefault— the opposite of the-soptionecs— the-Ceoptionfast— the-Foptionfull— the-foptioninteractive— the-Ioptionlex-compat— the-loptionmeta-ecs— the-Cmoptionperf-report— the-poptionread— the-Croptionstdout— the-toptionverbose— the-voptionwarn— the opposite of the-woption (use"%option nowarn"for-w)array— equivalent to"%array"pointer— equivalent to"%pointer"(this is the default)
💖 Some %options that unlock features you can't get any other way 💖
always-interactive— tells flex to generate a scanner that always considers its input "interactive". Normally, on each new input file, the scanner callsisatty()to try to figure out whether the scanner's input source is interactive and should therefore be read a character at a time. When this option is used, though, no such call is made. 🚫📞main— directs flex to provide a defaultmain()program for the scanner, which simply callsyylex(). This option impliesnoyywrap(see below).never-interactive— tells flex to generate a scanner that never considers its input "interactive" (again, no call made toisatty()). This is the opposite ofalways-interactive.stack— enables the use of start condition stacks (see Start Conditions above). 📚stdinit— if set (i.e.,%option stdinit), initializesyyinandyyouttostdinandstdout, instead of the default ofnil. Some existing lex programs depend on this behavior, even though it's not compliant with ANSI C, which does not requirestdinandstdoutto be compile-time constant. 😬yylineno— directs flex to generate a scanner that maintains the number of the current line read from its input in the global variableyylineno. This option is implied by%option lex-compat.
okay so bestie, about yywrap — if it's unset (that's %option noyywrap energy), it makes the scanner not call yywrap() when it hits an end-of-file. Instead it just assumes there are no more files to scan. Periodt. Until you, the user, point yyin at a new file and call yylex() again. 💅
flex literally scans your rule actions to figure out whether you're using the REJECT or yymore() features. The reject and yymore options are there to override its decision. You can set them (e.g., %option reject) to say "yes, I'm using this feature", or unset them (e.g., %option noyymore) to say "nope, actually I'm not using it". It's giving override energy. ✨
Three options take string-delimited values, offset with '='. So:
%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 (that's the -+ 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 bestie, so the whole entire point of flex's existence? it's designed to generate high-performance scanners. periodt. it's been optimized specifically for dealing well with large sets of rules, so if you're out here with a giant ruleset, flex has your back. 🫶
but now the tea ☕ — aside from the effects on scanner speed from those table compression -C options we already talked about above, there are a whole bunch of options/actions that will degrade performance. 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()so the first three up there? genuinely expensive, like they will tank your vibe. the last two? pretty cheap, ngl, they're basically fine.
also worth clocking: unput() is implemented as a routine call that potentially does quite a bit of work, while yyless() is a quite-cheap macro. so if all you're doing is putting back some excess text you scanned, use yyless() — don't be dramatic about it. 💅
REJECT should be avoided at all costs when performance is important. like fr fr, it is a particularly expensive option. no cap.
✨ getting rid of backing up (the messy part) ✨
so removing backing up is messy and often may be an enormous amount of work for a complicated scanner. but in principal, you begin by using the -b flag to generate a lex.backup file. for example, on this input:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;the file looks like this:
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.so let's decode this together bestie 🧚 the first few lines tell us there's a scanner state where it can make a transition on an 'o' but not on any other character, and that in that state the currently scanned text doesn't match any rule. this state happens when it's trying to match the rules found at lines 2 and 3 in the input file. so 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 that's matched. with a bit of headscratching (iykyk 😭) you can see this must be the state it's in when it has seen "fo". when that's happened, if anything other than another 'o' shows up, the scanner will have to back up to simply match the 'f' (by the default rule).
the comment about State #8 is telling us 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". and similarly, the comment for State #9 is about when "fooba" has been scanned and an 'r' does not follow. 💀
and that final comment? it's the lil reminder that there's no point going to all the trouble of removing backing up from the rules unless you're using -Cf or -CF, because there's no performance gain doing so with compressed scanners. don't waste your time, bestie.
🌸 how to actually fix it 🌸
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;
}you can also eliminate backing up among a list of keywords using a "catch-all" rule:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
[a-z]+ return TOK_ID;and honestly? this is usually the best solution when appropriate. ✨
ngl, backing up messages tend to cascade. with a complicated set of rules it's not uncommon to get hundreds of messages. but if one can decipher them, 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, so fingers crossed bestie 🤞)
important: 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. zero. nada. 😭
🦋 variable trailing context 🦋
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 as:
%%
mouse/cat|dog run();
rat/cat|dog run();or as:
%%
mouse|rat/cat run();
mouse|rat/dog run();and note that here the special '|' action does not provide any savings, and can even make things worse (see Deficiencies / Bugs below). so like, don't do that. 💅
👑 longer tokens = faster scanner 👑
another area where you (yes you, main character) can increase a scanner's performance — and one that's easier to implement</s> — comes 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 it doesn't often have to go through the additional work of setting up the scanning environment (e.g., yytext) for the action. slay. ✨
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);so 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 here's the iconic part: 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 '|'. so go off, add all the rules you want. 💅
🍓 one final example (in speeding up a scanner) 🍓
suppose you want to scan through a file containing identifiers and keywords, one per line, 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 */but be careful here, because we have now reintroduced backing up into the scanner. 😭 specifically: while we know 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. so 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 know 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. iconic behavior, honestly 🌈
💜 a couple more final notes 💜
- NUL's are slow. 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.
- the resize thing. another final note regarding performance: as mentioned above in the section How the Input is Matched, dynamically resizing
yytextto accommodate huge tokens is a slow process because it presently requires that the (huge) token be rescanned from the beginning. so 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. 🎀
so yeah, keep your tokens big but not too big, bestie. balance. 🫶
✨ GENERATING C++ SCANNERS ✨
okay bestie so you want C++?? flex said say less 💅 and there are literally two whole ways to make it happen.
Way one: just compile a scanner that flex generated using a C++ compiler instead of a C compiler. 🎀 And like, you should not hit any compilation errors (if you do find some, please report them to the email address given in the Author section below — she wants to know!). Then you can use C++ code in your rule actions instead of C code. Heads up though: the default input source for your scanner is still yyin, and default echoing is still done to yyout. Both of these remain FILE * variables and not C++ streams. 💀
Way two: you can also use flex to generate a C++ scanner class, using the -+ option (or, equivalently, %option c++), which is automatically specified if the name of the flex executable ends in a +, such as flex++. ✨ When 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
The first class, FlexLexer, provides an abstract base class defining the general scanner class interface. She provides the following member functions:
const char* YYText()— returns the text of the most recently matched token, the equivalent ofyytext. ✨int YYLeng()— returns the length of the most recently matched token, the equivalent ofyyleng.int lineno() const— returns the current input line number (see%option yylineno), or1if%option yylinenowas not used.void set_debug( int flag )— sets the debugging flag for the scanner, equivalent to assigning toyy_flex_debug(see the Options section above). Note that you must build the scanner using%option debugto 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 istream* object pointer and not a FILE*), yy_flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is an istream* object pointer).
👑 the second class: yyFlexLexer
The second class defined in FlexLexer.h is yyFlexLexer, which is derived from FlexLexer. It defines the following additional member functions:
yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )— constructs ayyFlexLexerobject using the given streams for input and output. If not specified, the streams default tocinandcout, respectively.virtual int yylex()— performs the same roleyylex()does for ordinary flex scanners: it scans the input stream, consuming tokens, until a rule's action returns a value. If you derive a subclassSfromyyFlexLexerand want to access the member functions and variables ofSinsideyylex(), then you need to use%option yyclass="S"to inform flex that you will be using that subclass instead ofyyFlexLexer. In this case, rather than generatingyyFlexLexer::yylex(), flex generatesS::yylex()(and also generates a dummyyyFlexLexer::yylex()that callsyyFlexLexer::LexerError()if called). 🔥virtual void switch_streams(istream* new_in = 0, ostream* new_out = 0)— reassignsyyintonew_in(if non-nil) andyyouttonew_out(ditto), deleting the previous input buffer ifyyinis reassigned.int yylex( istream* new_in, ostream* new_out = 0 )— first switches the input streams viaswitch_streams( new_in, new_out )and then returns the value ofyylex().
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 tomax_sizecharacters intobufand returns the number of characters read. To indicate end-of-input, return0characters. Note that "interactive" scanners (see the-Band-Iflags) define the macroYY_INTERACTIVE. If you redefineLexerInput()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 outsizecharacters from the bufferbuf, 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 streamcerrand 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. so like, don't be shocked when it does 💀
💀 INCOMPATIBILITIES WITH LEX AND POSIX 💀
okay bestie, sit down, we're doing lore. flex is a full-on rewrite of the AT&T Unix lex tool (they share literally zero code, wild, I know), but it comes with some extensions and some incompatibilities, and both of those matter if you're trying to write a scanner that plays nice with either implementation. flex is fully compliant with the POSIX lex specification, with exactly one exception: when you're using %pointer (which is the default btw), calling unput() destroys the contents of yytext, and that is counter to the POSIX spec. 😭
So in this section we're gonna yap about every known area 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, and the price is a major loss in your generated scanner's performance. We'll note below which incompatibilities get fixed by using -l.
Flex is fully compatible with lex except for these, bestie:
- The undocumented lex scanner internal variable
yylinenois not supported unless you use-lor%option yylineno.
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. Ifinput()encounters an end-of-file the normalyywrap()processing is done. Arealend-of-file is returned byinput()asEOF.
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 this message:
fatal flex scanner internal error--end of buffer missedTo 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 theECHOmacro is done to the file-pointeryyout(defaultstdout).
output() is not part of the POSIX specification.
- lex does not support exclusive start conditions (
%x), though they are in the POSIX specification. - When definitions are expanded, flex encloses them in 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.
- The lex
%r(generate a Ratfor scanner) option is not supported. It is not part of the POSIX specification. - After a call to
unput(),yytextis 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-loption 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
%asupported by lex are not required by flex scanners; flex ignores them. - The name
FLEX_SCANNERis#define'd so scanners may be written for use with either flex or lex. Scanners also includeYY_FLEX_MAJOR_VERSIONandYY_FLEX_MINOR_VERSIONindicating which version of flex generated the scanner (for example, for the 2.5 release, these defines would be2and5respectively).
And now the reverse tea ☕ — these 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 friendsyyterminate()yy_set_interactive()yy_set_bol()YY_AT_BOL()<<EOF>><*>YY_DECLYY_STARTYY_USER_ACTIONYY_USER_INIT#linedirectives%{}'s around actions- multiple actions on a line
plus almost all of the flex flags. 👑
That last feature in the list is about how, 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.
🩺 DIAGNOSTICS 🩺
warning, rule cannot be matched— this means 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— this means it's 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-swas given, presumably this is not intended. 🙈
reject_used_but_not_detected undefinedoryymore_used_but_not_detected undefined— these errors can occur at compile time. They indicate that the scanner usesREJECToryymore()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#includefile, for example). Use%option rejector%option yymoreto indicate to flex that you really do use these features.
flex scanner jammed— a scanner compiled with-shas 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%arrayand one of its rules matched a string longer than theYYLMAXconstant (8K bytes by default). You can increase the value by#define'ingYYLMAXin 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-8flag, and your scanner defaulted to 7-bit because you used the-Cfor-CFtable compression options. See the discussion of the-7flag for details.
flex scanner push-back overflow— you usedunput()to push back so much text that the scanner's buffer could not hold both the pushed-back text and the current token inyytext. 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 useREJECT.
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).
📁 FILES 📁
-lfl— library with which scanners must be linked.lex.yy.c— generated scanner (calledlexyy.con 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-bflag (calledlex.bckon some systems).
💀 DEFICIENCIES / BUGS 💀
Okay bestie, this is the part where flex is like "I'm not perfect and I'm gonna be so real with you rn" 🙈
- Some trailing context patterns cannot be properly matched, and instead of quietly suffering they generate warning messages, specifically the iconic
"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. (Heads up: the POSIX draft says the text matched by such patterns is undefined, so like... don't rely on it, periodt.) - For some trailing context rules, parts that are actually fixed-length don't get recognized as fixed-length, which leads to that performance loss we talked about earlier 😭. Specifically, parts using
'|'or{n}(like"foo{3}") are always considered variable-length. No cap. - Combining trailing context with the special
'|'action can result in fixed trailing context getting turned into the more expensive variable trailing context. For example, in the following:
%%
abc |
xyz/def- Use of
unput()invalidatesyytextandyyleng, unless the%arraydirective or the-loption has been used. 🫠 - Pattern-matching of NUL's is substantially slower than matching other characters. Just so you know.
- Dynamic resizing of the input buffer is slow, because it entails rescanning all the text matched so far by the current (and generally huge) token. It's giving "why is this taking so long" 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. Callinput()instead. That's the move. 💅 - The total table entries listed by the
-vflag excludes the number of table entries needed to determine what rule has been matched. That number of entries is equal to the number of DFA states if the scanner does not useREJECT, and somewhat greater than the number of states if it does. REJECTcannot be used with the-for-Foptions. 🚫- The flex internal algorithms need documentation. (Yeah. That's the bug. It's just... undocumented. Iconic of them honestly.)
👀 SEE ALSO 👀
lex(1),yacc(1),sed(1),awk(1).- John Levine, Tony Mason, and Doug Brown, Lex & Yacc, O'Reilly and Associates. Be sure to get the 2nd edition. (they mean it, bestie)
- 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). 🧠✨
👑 AUTHOR 👑
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 Deraadt, 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. 😭💖 (the way that this list ate, fr fr)
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. 🫠
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. 🌈
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. 📧
and that's a wrap on the whole flex(1) page bestie!! 🎀 This was Version 2.5, dated April 1995, and you just read the one and only FLEX(1) manual page — front to back, no skimming, main character behavior only 👑💅 Go forth and lex, my love. ✨🦋
📦 where this page comes from
- fedora 43: compat-flex 2.5.4a-33.fc41 · BSD
- fedora 44: compat-flex 2.5.4a-33.fc41 · BSD
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms