The regex-centric, fast and flexible scanner generator for C++. RE/flex lexical analyzers are compatible with Flex and Bison. Includes a fast stand-alone regex engine library. Adds Boost 💪
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Robert van Engelen
Latest commit c7c4c67 Aug 21, 2018


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"The regex-centric, fast and flexible lexical analyzer generator for C++"

Flex reimagined. Fast, flexible, adds Boost 💪

RE/flex is faster than Flex while providing a wealth of new features. RE/flex is also much faster than regex libraries such as Boost.Regex, C++11 std::regex, PCRE2 and RE2. For example, tokenizing a representative C source code file into 244 tokens takes only 13 microseconds:

Command / FunctionSoftwareTime (μs)
reflex --fastRE/flex13
flex -+ --fullFlex17
reflex --fullRE/flex29
reflex -m=boost-perlBoost.Regex230
pcre2_match()PCRE2 (pre-compiled)318
reflex -m=boostBoost.Regex POSIX mode450
flex -+Flex3968
RE2::Consume()RE2 (pre-compiled)5088
RE2::Consume()RE2 POSIX mode (pre-compiled)5420
std::cregex_iterator()C++11 std::regex14784

Note: Best times of 10 tests with average time in microseconds over 100 runs (using clang 8.0.0 with -O2, 2.9 GHz Intel Core i7, 16 GB 2133 MHz LPDDR3).


  • Compatible with Flex to eliminate a learning curve, making a transition to RE/flex frustration-free.
  • Includes many examples, such as a tokenizer for C/C++ code, a tokenizer for Python code, a tokenizer for Java code, and more.
  • Works with Bison and supports reentrant, bison-bridge, bison-locations and Bison 3.0 C++ interface %skeleton "".
  • Extensive documentation in the online User Guide.
  • Adds Unicode support with Unicode property matching \p{C} and C++11, Java, C#, and Python Unicode properties for identifier name matching.
  • Adds indent/nodent/dedent anchors to match text with indentation, including \t (tab) adjustments.
  • Adds lazy quantifiers to the POSIX regular expression syntax, so hacks are no longer needed to work around greedy repetitions in Flex.
  • Adds word boundary anchors to the POSIX regular expression syntax.
  • Adds an extensible hierarchy of pattern matcher engines, with a choice of regex engines, including the RE/flex regex engine and Boost.Regex.
  • Adds freespace mode option to improve readability of lexer specifications.
  • Adds %class and %init to customize the generated Lexer classes.
  • Adds %include to modularize lexer specifications.
  • Generates clean source code that defines an MT-safe (reentrant) C++ Lexer class derived from an abstract lexer class template, parameterized by matcher class type.
  • Multiple lexer classes can be combined and used in one application.
  • Configurable Lexer class generation to customize the interface for various parsers, including Yacc and Bison.
  • Generates scanners for lexical analysis on files, C++ streams, and (wide) strings, with automatic fast conversion of UTF-16/32 to UTF-8 for matching Unicode on UTF-encoded input files.
  • Generates lex.yy.cpp files while Flex generates files (in C++ mode with flex option -+), to distinguish the generated files.
  • Generates Graphviz files to visualize FSMs with the Graphviz dot tool.
  • Conversion of regex expressions, for regex engines that lack regex features.
  • The RE/flex regex library makes C++11 std::regex and Boost.Regex much easier to use in plain C++ code for pattern matching on (wide) strings, files, and streams.

The RE/flex software is fully self-contained. No other libraries are required. Boost.Regex is optional to use as a regex engine.

The RE/flex repo includes tokenizers for Java, Python, and C/C++.


Windows users

Use reflex/bin/reflex.exe from the command line or add a Custom Build Step in MSVC++ as follows:

  1. select the project name in Solution Explorer then Property Pages from the View menu (see also custom-build steps in Visual Studio);

  2. add an extra path to the reflex/include folder in the Include Directories under VC++ Directories, which should look like $(VC_IncludePath);$(WindowsSDK_IncludePath);C:\Users\YourUserName\Documents\reflex\include (this assumes the reflex source package is in your Documents folder).

  3. enter "C:\Users\YourUserName\Documents\reflex\bin\reflex.exe" --header-file "C:\Users\YourUserName\Documents\mylexer.l" in the Command Line property under Custom Build Step (this assumes mylexer.l is in your Documents folder);

  4. enter lex.yy.h lex.yy.cpp in the Outputs property;

  5. specify Execute Before as PreBuildEvent.

If you are using specific reflex options such as --flex then add these in step 3.

Before compiling your program with MSVC++, drag the folders reflex/lib and reflex/unicode to the Source Files in the Solution Explorer panel of your project. Next, run reflex.exe simply by compiling your project (which may fail, but that is OK for now as long as we executed the custom build step to run reflex.exe). Drag the generated lex.yy.h and lex.yy.cpp files to the Source Files. Now you are all set!

Unix/Linux and Mac OS X

You have two options: 1) quick install or 2) configure and make.

Quick install

A quick clean build, assuming your environment is pretty much standard:

$ ./
$ ./

Or use the make -f Make command to do the same:

$ cd src; make -f Make

This compiles the reflex tool and installs it locally in reflex/bin. For local use of RE/flex in your project, you can add this location to your $PATH variable to enable the new reflex command:

export PATH=$PATH:/your_path_to_reflex/reflex/bin

Note that the libreflex.a and libraries are saved locally in reflex/lib. Link against the library when you use the RE/flex regex engine in your code, such as:

$ c++ <options and .o/.cpp files> -L/your_path_to_reflex/reflex/lib -lreflex

or you could statically link libreflex.a with:

$ c++ <options and .o/.cpp files> /your_path_to_reflex/reflex/lib/libreflex.a

Also note that the RE/flex header files that you will need to include in your project are locally located in include/reflex.

To fully install the library and the reflex command in /usr/local/lib and /usr/local/bin:

$ sudo ./

Or use the make -f Make install command to do the same:

$ cd lib; sudo make -f Make install
$ cd src; sudo make -f Make install

Configure and make

The configure script accepts configuration and installation options. To view these options, run:

$ ./configure --help

Run configure and make:

$ ./configure && make

After this successfully completes, you can optionally run make install to install the reflex command and libreflex library:

$ sudo make install

Unfortunately, cloning from Git does not preserve timestamps which means that you may run into "WARNING: 'aclocal-1.15' is missing on your system." To work around this problem, run:

$ autoreconf -fi
$ ./configure && make

Optional libraries to install

  • To use Boost.Regex as a regex engine with the RE/flex library and scanner generator, install Boost and link your code against libboost_regex.a

  • To visualize the FSM graphs generated with reflex option --graphs-file, install Graphviz dot.


There are two ways you can use this project:

  1. as a scanner generator for C++, similar to Flex;
  2. as an extensible regex matching library for C++.

For the first option, simply build the reflex tool and run it on the command line on a lexer specification:

$ reflex --flex --bison --graphs-file lexspec.l

This generates a scanner for Bison from the lexer specification lexspec.l and saves the finite state machine (FSM) as a Graphviz .gv file that can be visualized with the Graphviz dot tool:

$ dot -Tpdf reflex.INITIAL.gv > reflex.INITIAL.pdf
$ open reflex.INITIAL.pdf

Visualize DFA graphs with Graphviz dot

Several examples are included to get you started. See the manual for more details.

For the second option, simply use the new RE/flex matcher classes to start pattern matching on strings, wide strings, files, and streams.

You can select matchers that are based on different regex engines:

  • RE/flex regex: #include <reflex/matcher.h> and use reflex::Matcher;
  • Boost.Regex: #include <reflex/boostmatcher.h> and use reflex::BoostMatcher or reflex::BoostPosixMatcher;
  • C++11 std::regex: #include <reflex/stdmatcher.h> and use reflex::StdMatcher or reflex::StdPosixMatcher.

Each matcher may differ in regex syntax features (see the full documentation), but they have the same methods and iterators:

  • matches() returns nonzero if the input matches the specified pattern;
  • find() search input and return nonzero if a match was found;
  • scan() scan input and return nonzero if input at current position matches;
  • split() return nonzero for a split of the input at the next match;
  • find.begin()...find.end() filter iterator;
  • scan.begin()...scan.end() tokenizer iterator;
  • split.begin()...split.end() splitter iterator.

For example:

#include <reflex/boostmatcher.h> // reflex::BoostMatcher, reflex::Input, boost::regex
// use a BoostMatcher to check if the birthdate string is a valid date
if (reflex::BoostMatcher("\\d{4}-\\d{2}-\\d{2}", birthdate).matches() != 0)
  std::cout << "Valid date!" << std::endl;

With a group capture to fetch the year:

#include <reflex/boostmatcher.h> // reflex::BoostMatcher, reflex::Input, boost::regex
// use a BoostMatcher to check if the birthdate string is a valid date
reflex::BoostMatcher matcher("(\\d{4})-\\d{2}-\\d{2}", birthdate);
if (matcher.matches() != 0)
  std::cout << std::string(matcher[1].first, matcher[1].second) << " was a good year!" << std::endl;

To search a string for words \w+:

#include <reflex/boostmatcher.h> // reflex::BoostMatcher, reflex::Input, boost::regex
// use a BoostMatcher to search for words in a sentence
reflex::BoostMatcher matcher("\\w+", "How now brown cow.");
while (matcher.find() != 0)
  std::cout << "Found " << matcher.text() << std::endl;

The split method is roughly the inverse of the find method and returns text located between matches. For example using non-word matching \W+:

#include <reflex/boostmatcher.h> // reflex::BoostMatcher, reflex::Input, boost::regex
// use a BoostMatcher to search for words in a sentence
reflex::BoostMatcher matcher("\\W+", "How now brown cow.");
while (matcher.split())
  std::cout << "Found " << matcher.text() << std::endl;

To pattern match the content of a file that may use UTF-8, 16, or 32 encodings:

#include <reflex/boostmatcher.h> // reflex::BoostMatcher, reflex::Input, boost::regex
// use a BoostMatcher to search and display words from a FILE
FILE *fd = fopen("somefile.txt", "r");
if (fd == NULL)
reflex::BoostMatcher matcher("\\w+", fd);
while (matcher.find())
  std::cout << "Found " << matcher.text() << std::endl;

Same again, but this time with a C++ input stream:

#include <reflex/boostmatcher.h> // reflex::BoostMatcher, reflex::Input, boost::regex
// use a BoostMatcher to search and display words from a stream
std::ifstream file("somefile.txt", std::ifstream::in);
reflex::BoostMatcher matcher("\\w+", file);
while (matcher.find())
  std::cout << "Found " << matcher.text() << std::endl;

Stuffing the search results into a container using RE/flex iterators:

#include <reflex/boostmatcher.h> // reflex::BoostMatcher, reflex::Input, boost::regex
#include <vector>         // std::vector
// use a BoostMatcher to convert words of a sentence into a string vector
reflex::BoostMatcher matcher("\\w+", "How now brown cow.");
std::vector<std::string> words(matcher.find.begin(), matcher.find.end());

Use C++11 range-based loops with RE/flex iterators:

#include <reflex/stdmatcher.h> // reflex::StdMatcher, reflex::Input, std::regex
// use a StdMatcher with std::regex to to search for words in a sentence
for (auto& match : reflex::StdMatcher("\\w+", "How now brown cow.").find)
  std::cout << "Found " << match.text() << std::endl;

RE/flex also allows you to convert expressive regex syntax forms such as \p Unicode classes, character class set operations such as [a-z--[aeiou]], escapes such as \X, and (?x) mode modifiers, to a regex string that the underlying regex library understands and will be able to use:

  • std::string reflex::Matcher::convert(const std::string& regex, reflex::convert_flag_type flags)
  • std::string reflex::BoostMatcher::convert(const std::string& regex, reflex::convert_flag_type flags)
  • std::string reflex::StdMatcher::convert(const std::string& regex, reflex::convert_flag_type flags)

For example:

#include <reflex/matcher.h> // reflex::Matcher, reflex::Input, reflex::Pattern
// use a Matcher to check if sentence is in Greek:
static const reflex::Pattern pattern(reflex::Matcher::convert("[\\p{Greek}\\p{Zs}\\pP]+", reflex::convert_flag::unicode));
if (reflex::Matcher(pattern, sentence).matches() != 0)
  std::cout << "This is Greek" << std::endl;

We use convert with optional flag reflex::convert_flag::unicode to make . (dot), \w, \s and so on match Unicode and to convert \p Unicode character classes.

Conversion is fast (it runs in linear time in the size of the regex), but it is not without some overhead. Making converted regex patterns static as shown above saves the cost of conversion to just once to support many matchings.

How to contribute?


Where do I find the documentation?

Read more about RE/flex in the manual.

License and copyright

RE/flex by Robert van Engelen, Genivia Inc. Copyright (c) 2015-2018, All rights reserved.

RE/flex is distributed under the BSD-3 license LICENSE.txt. Use, modification, and distribution are subject to the BSD-3 license.


  • Nov 14, 2016: 0.9.0 beta released
  • Nov 15, 2016: 0.9.1 improved portability
  • Nov 17, 2016: 0.9.2 improvements and fixes for minor issues
  • Nov 19, 2016: 0.9.3 replaces %import with %include, adds freespace option -x, fixes minor issues
  • Nov 20, 2016: 0.9.4 fixes minor issues, added new examples/json.l
  • Nov 25, 2016: 0.9.5 bug fixes and improvements
  • Dec 1, 2016: 0.9.6 portability improvements
  • Dec 6, 2016: 0.9.7 bug fixes, added option --regexp-file, Python tokenizer
  • Dec 9, 2016: 0.9.8 fixes minor issues, improved reflex tool options --full and --fast, generates scanner with FSM table or a fast scanner with FSM code, respectively
  • Jan 8, 2017: 0.9.9 bug fixes and improved Flex compatibility
  • Jan 15, 2017: 0.9.10 improved compatibility with Flex options, fixed critical issue with range unions
  • Jan 25, 2017: 0.9.11 added C++11 std::regex matching engine support, moved .h files to include/reflex, requires #include <reflex/xyz.h> from now on, fixed errno_t portability issue
  • Mar 3, 2017: 0.9.12 refactored and improved, includes new regex converters for regex engines that lack regex features such as Unicode character classes
  • Mar 4, 2017: 0.9.13 improved warning and error messages
  • Mar 6, 2017: 0.9.14 reflex option -v shows stats with execution timings, bug fixes
  • Mar 8, 2017: 0.9.15 added wtext(), wpair(), winput() methods, other improvements
  • Mar 22, 2017: 0.9.16 bug fixes, speed improvements, improved option --unicode regex conversion, also with (?u:), changed wtext() to wstr() and added a str() method
  • Mar 24, 2017: 0.9.17 improvements
  • Mar 26, 2017: 0.9.18 added reflex option -p (--perf-report) for performance debugging, added doc/man/reflex.1 man page, added interactive readline example
  • Mar 31, 2017: 0.9.19 fixed reflex option -m, now resets the lexer, fixed reassigning the same input to the lexer that caused UTF BOM to be read twice
  • Apr 5, 2017: 0.9.20 EBCDIC file translation, other improvements
  • Apr 10, 2017: 0.9.21 fixed option -P to support multiple lexer classes in one application, added configure installation script, optional quick install with (renamed from
  • Apr 12, 2017: 0.9.22 improved explanations of matches(), find(), scan(), split() that return nonzero for a match, other minor improvements
  • May 24, 2017: 0.9.23 improved portability, added file encoding conversions for CP-1250 to CP-1258, CP 437, and CP 850/858
  • Jun 24, 2017: 0.9.24 added an option for users to define their own custom code pages to translate input, fixed # in free space mode
  • Jun 28, 2017: 0.9.25 fixed --fast FSM not always halting on EOF after a mismatch; fixed buffer realloc, added new examples/csv.l
  • Jul 5, 2017: 0.9.26 fixed wstr() always returning UTF-16 strings (should be UTF-16 only when std::wstring requires it)
  • Sep 26, 2017: 0.9.27 the Flex-compatible yy_scan_string(), yy_scan_bytes() and yy_scan_buffer() functions now create a new buffer as in Flex, delete this buffer with yy_delete_buffer(); fixed examples to work with newer Bison versions (Bison 3.0.4)
  • Dec 12, 2017: 0.9.28 added yy_scan_wstring and yy_scan_wbuffer for wide string scanning with Flex-like functions
  • Jan 28, 2018: 1.0.0 removed dynamic exception specifications to comply with C++17, upgraded to stable release 1.0
  • Feb 24, 2018: 1.0.1 added Unicode IsBlockName categories
  • Mar 6, 2018: 1.0.2 added namespace nesting with %option namespace=NAME1.NAME2.NAME3 ...
  • Mar 7, 2018: 1.0.3 fixed --namespace and %option namespace
  • Apr 22, 2018: 1.0.4 updated to Unicode 10, cleaned up code to remove tool warnings
  • Jun 29, 2018: 1.0.5 updated --namespace for options --fast and --full to support the generation of multiple optimized lexers placed in namespaces.
  • Jul 9, 2018: 1.0.6 added --bison-cc option to generate scanners for Bison 3.0 %skeleton "" C++ parsers, included two examples flexexample9xx and reflexexample9xx to demo this feature.
  • Jul 12, 2018: 1.0.7 added --bison-cc-namespace and --bison-cc-parser options to customize Bison 3.0 %skeleton "" C++ parsers.
  • Jul 30, 2018: 1.0.8 updated to Unicode 11.
  • Aug 21, 2018: 1.0.9 fixed reflex regex library matching with range quantifiers by correcting coding typo.