flex Lexical Analyzer Generator

If you have downloaded a release tar archive (e.g., flex-<version>.tar.gz), you can build Flex using the standard GNU autotools workflow.

1. Run ./configure with any necessary options (use --help to see available options).
2. Run make to compile the project.
3. (Optional) Run make check to execute the test suite.
4. Run make install to install the binaries.

Note: You can use the --prefix argument during the configure step or the prefix argument during make install to specify a custom installation directory. You can also use the DESTDIR argument during make install for staged installations.

$ ./configure <any configure options you need>
$ make
$ make check
$ make install

The examples/fastwc directory provides several implementations of a wc tool (counting lines, words, and characters) to demonstrate performance scaling in flex:

• wc1.l: A naive implementation.
• wc2.l: Faster; uses rules to match multiple tokens at once.
• wc3.l: Faster; adds rules to match longer runs of tokens.
• wc4.l: The fastest flex implementation; uses extensive rules to maximize matching length.
• wc5.l: Demonstrates the performance penalty of backing up (slower than wc3.l).
• mywc.c: A hand-coded C implementation for comparison.

If you need to create multiple different lexer classes in the same C++ project, you must rename the generated yyFlexLexer class for each one using the -P flag during the flex generation phase. To include the header for different lexer types in the same translation unit, you must undefine and redefine yyFlexLexer before each inclusion.

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

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

To compile all the example programs provided in the manual, use the make command targeting the Makefile.examples file. This requires flex and gcc to be installed on your system.

make -f Makefile.examples

The flex command-line tool is used to generate fast lexical analyzers (scanners) from a specification file. It processes rules and regular expressions to produce C or C++ code.

Basic usage involves passing the specification file as an argument. You can control the output filename, prefix, and various scanner behaviors using command-line flags.

To achieve high performance in a flex scanner, follow these principles:

1. Match as much text as possible with each rule: Instead of having many small rules, combine them into larger rules that match longer sequences of tokens at once. Adding more rules to match longer runs does not inherently slow down the scanner.
2. Avoid backing up: Ensure your rules do not require the scanner to move the input pointer backward. Backing up is expensive and significantly degrades performance.
3. Use compression options: When compiling with flex, using the -Cf flag (disabling table compression) can improve performance compared to the default -Cem compression.

Warning: These techniques increase performance at the cost of decreased maintainability. Only apply them when performance is a critical requirement.