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lexical analyzer (1) regex (1) flex (1) lexer (1) compiler (1)
Robert van Engelen

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A high-performance C++ regex library and lexical analyzer generator with Unicode support. Extends Flex++ with Unicode, indent/dedent anchors, lazy quantifiers, functions for lex and syntax error reporting and more. The RE/flex lexical analyzer generator accepts Flex lexer specifications and seamlessly integrates with Bison parsers.

See Constructing Lexical Analyzers with RE/flex - a Modern Alternative to Flex for C++ and the RE/flex User Guide for more details.

Features

  • Compatible with Flex to eliminate a learning curve, making a transition from Flex++ to RE/flex frustration-free.
  • Generates reusable source code that is easy to understand.
  • Integrates seamlessly with Bison and supports reentrant, bison-bridge, bison-locations, Bison 3.0 C++ interface %skeleton "lalr1.cc" and Bison complete symbols.
  • Fully supports Unicode and Unicode properties \p{C}, including Unicode identifier matching for C++11, Java, C#, and Python source code.
  • Auto-detects UTF-8/16/32 input to match Unicode patterns.
  • Supports file encodings ISO-8859-1 through ISO-8859-15, CP 1250 through 1258, CP 437, CP 850, CP 858, KOI8, MACROMAN, EBCDIC, and custom code pages.
  • Generates scanners for lexical analysis on files, C++ streams, (wide) strings, and memory such as mmap files.
  • IIncludes many examples, such as a mini C compiler to Java bytecode, a tokenizer for C/C++ source code, a tokenizer for Python source code, a tokenizer for Java source code, and more.
  • Extensive documentation in the online User Guide.
  • Indent/nodent/dedent anchors to match text with indentation, including custom \t (tab) widths.
  • Lazy quantifiers, no hacks are needed to work around greedy repetitions.
  • Word boundary anchors.
  • Freespace mode option to improve readability of lexer specifications.
  • %class and %init to customize the generated Lexer classes.
  • %include to modularize lexer specifications.
  • Multiple lexer classes can be combined and used in one application, e.g. by multiple threads in a thread-safe manner.
  • Configurable Lexer class generation to customize the interface for various parsers, including Yacc and Bison.
  • Generates Graphviz files to visualize FSMs with the Graphviz dot tool.
  • Includes an extensible hierarchy of pattern matcher engines, with a choice of regex engines, including the RE/flex regex engine, PCRE2, and Boost.Regex.
  • The RE/flex regex library makes C++11 std::regex, PCRE2, and Boost.Regex much easier to use for pattern matching on (wide) strings, files, and streams.
  • Lots of other improvements over Flex++, such as yypush_buffer_state saves the scanner state (line, column, and indentation positions), not just the input buffer; no input buffer length limit (Flex has a 16KB limit); line() returns the current line (e.g. for error reporting)

The RE/flex software is self-contained. No other libraries are required. PCRE2 and Boost.Regex are optional to use as regex engines.

The RE/flex repo includes tokenizers for Java, Python, C/C++, JSON, XML, YAML.

Installation

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.

To compile your program with MSVC++, make sure to drag the folders reflex/lib and reflex/unicode to the Source Files in the Solution Explorer panel of your project. After running reflex.exe drag the generated lex.yy.h and lex.yy.cpp files there as well. If you are using specific reflex command-line options such as --flex, add these in step 3.

Unix/Linux and Mac OS

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

Quick install

For a quick clean build assuming your environment is pretty much standard:

$ ./clean.sh
$ ./build.sh

This compiles the reflex tool and installs it locally in reflex/bin. You can add this location to your $PATH variable to enable the new reflex command:

export PATH=$PATH:/reflex_install_path/bin

The libreflex.a and libreflex.so libraries are saved locally in lib. Link against one of these libraries when you use the RE/flex regex engine in your code. The RE/flex header files are locally located in include/reflex.

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

$ sudo ./allinstall.sh

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

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.

Usage

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 lex specification:

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

This generates a scanner for Bison from the Flex 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 from begin to end matches;
  • 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() != 0)
  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)
  exit(EXIT_FAILURE);
reflex::BoostMatcher matcher("\\w+", fd);
while (matcher.find() != 0)
  std::cout << "Found " << matcher.text() << std::endl;
fclose(fd);

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() != 0)
  std::cout << "Found " << matcher.text() << std::endl;
file.close();

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 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)
  • std::string reflex::BoostMatcher::convert(const std::string& regex)
  • std::string reflex::StdMatcher::convert(const std::string& regex)

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]+"));
if (reflex::Matcher(pattern, sentence).matches() != 0)
  std::cout << "This is Greek" << std::endl;

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.

You can use convert with option reflex::convert_flag::unicode to make . (dot), \w, \s and so on match Unicode.

RE/flex by Robert van Engelen, Genivia Inc.
Copyright (c) 2015-2017, 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.