flex

Generate an analyzer from a Lex file (output: lex.yy.c)

$ flex [analyzer.l]

$ flex -o [analyzer.c] [analyzer.l]

$ flex -t [analyzer.l]

$ flex -B [analyzer.l]

$ flex -i [analyzer.l]

$ flex -d [analyzer.l]

$ cc lex.yy.c -o [executable] -lfl

flex [options] [file...]

Flex (fast lexical analyzer generator) generates programs that perform pattern-matching on text. Given a specification file describing patterns and actions (typically with .l extension), flex generates C source code implementing a lexical analyzer.
The generated scanner reads input, matches it against patterns defined in the specification, and executes associated actions. This is commonly used to build tokenizers for compilers and interpreters, working alongside parser generators like bison or yacc.
By default, flex generates output to lex.yy.c. The generated code requires linking with -lfl (flex library) or defining yywrap() to return 1.

-o, --outfile file

Write generated scanner to specified file

Write scanner to stdout instead of file

Generate batch scanner (optimized for throughput)

Generate interactive scanner (default)

Generate case-insensitive scanner

Enable debug mode in generated scanner

Generate backup information file (lex.backup)

Generate performance report to stderr

Print scanner statistics to stderr

Generate fast, large scanner (no table compression)

Generate fast scanner with table compression

Generate C header file

Generate C++ scanner class

Track line numbers in generated scanner

The generated scanner depends on the flex runtime library (-lfl) unless %option noyywrap is specified. Large pattern sets may produce sizable table-driven scanners; use compression options (-Ce, -Cm) to reduce size at some performance cost. The default interactive mode adds buffering overhead; use -B for batch processing.

Flex was written by Vern Paxson around 1987 as a free, open-source replacement for the original lex utility from AT&T Unix (created by Mike Lesk and Eric Schmidt in the mid-1970s). The name stands for "fast lexical analyzer generator." Flex became the standard lex implementation on most Unix-like systems and continues to be maintained on GitHub.

bison(1), yacc(1), gcc(1), cc(1)