RE/flex

RE/flex (regex-centric, fast lexical analyzer) is a free and open source computer program written in C++ that generates fast lexical analyzers (also known as "scanners" or "lexers")  in C++. RE/flex offers full Unicode support, indentation anchors, word boundaries, lazy quantifiers (non-greedy, lazy repeats), and performance tuning options. RE/flex accepts Flex lexer specifications and offers options to generate scanners for Bison parsers. RE/flex includes a fast C++ regular expression library.

History
The RE/flex project was designed and implemented by professor Robert van Engelen in 2016 and released as free open source. The software evolved with several contributions made by others. The RE/flex tool generates lexical analyzers based on regular expression ("regex") libraries, instead of fixed DFA tables generated by traditional lexical analyzer generators.

Lexer specification
The RE/flex lexical analyzer generator accepts an extended syntax of Flex lexer specifications as input. The RE/flex specification syntax is more expressive than the traditional Flex lexer specification syntax and may include indentation anchors, word boundaries, lazy quantifiers (non-greedy, lazy repeats), and new actions such as  to retrieve Unicode wide-string matches.

A lexer specification is of the form:

The Definitions section includes declarations and customization options, followed by name-pattern pairs to define names for regular expression patterns. Named patterns may be referenced in other patterns by embracing them in  and. The following example defines two names for two patterns, where the second pattern  uses the previously named pattern  :

The Rules section defines pattern-action pairs. The following example defines a rule to translate a number to the lexer class integer  member:

The User Code section typically defines C/C++ functions, for example a  program:

The  class is generated by RE/flex as instructed by the   directive in the lexer specification. The generated  source code contains the algorithm for lexical analysis, which is linked with the   library.

Source code output
The generated algorithm for lexical analysis is based on the concept that any regular expression engine can in principle be used to tokenize input into tokens: given a set of $n$ regular expression patterns $$p_i$$ for $$i = 1,\ldots,n$$, a regular expression of the form  with $n$ alternations may be specified to match and tokenize the input. In this way, the group capture index $i$ of a matching pattern $$p_i$$ that is returned by the regular expression matcher identifies the pattern $$p_i$$ that matched the input text partially and continuously after the previous match.

This approach makes it possible for any regex library that supports group captures to be utilized as a matcher. However, note that all groupings of the form  in patterns must be converted to non-capturing groups of the form   to avoid any unwanted group capturing within sub-expressions.

The following RE/flex-generated  class   method repeatedly invokes the matcher's   (continuous partial matching) operation to tokenize input:

If none of the $n$ patterns match and the end-of-file (EOF) is not reached, the so-called "default rule" is invoked. The default rule echo's the current input character and advances the scanner to the next character in the input.

The regular expression pattern  is produced by RE/flex from a lexer specification with $n$ rules of pattern-action pairs:

From this specification, RE/flex generates the aforementioned  class with the   method that executes actions corresponding to the patterns matched in the input. The generated  class is used in a C++ application, such as a parser, to tokenize the input into the integer-valued tokens returned by the actions in the lexer specification. For example:

Note that  returns an integer value when an action executes. Otherwise,  does not return a value and continues scanning the input, which is often used by rules that ignore input such as comments.

This example tokenizes a file. A lexical analyzer often serves as a tokenizer for a parser generated by a parser generator such as Bison.

Compatibility
RE/flex is compatible with Flex specifications when  is used. This generates a  class with   method. RE/flex is also compatible with Bison using a range of RE/flex options for complete coverage of Bison options and features.

By contrast to Flex, RE/flex scanners are thread-safe by default on work with reentrant Bison parsers.

Unicode support
RE/flex supports Unicode regular expression patterns in lexer specifications and automatically tokenizes UTF-8, UTF-16, and UTF-32 input files. Code pages may be specified to tokenize input files encoded in ISO/IEC 8859 1 to 16, Windows-1250 to Windows-1258, CP-437, CP-850, CP-858, MacRoman, KOI-8, EBCDIC, and so on. Normalization to UTF-8 is automatically performed by internal incremental buffering for (partial) pattern matching with Unicode regular expression patterns.

Indent, nodent, and dedent matching
RE/flex integrates indent and dedent matching directly in the regular expression syntax with new  and   anchors. These indentation anchors detect changes of line indentation in the input. This allows many practical scenarios to be covered to tokenize programming languages with indented block structures. For example, the following lexer specification detects and reports indentation changes:

Lazy quantifiers
Lazy quantifiers may be associated with repeats in RE/flex regular expression patterns to simplify the expressions using non-greedy repeats, when applicable. Normally matching is "greedy", meaning that the longest pattern is matched. For example, the pattern   with the greedy   repeat matches , but also matches   because   matches any characters except newline and   is longer than. Using a lazy quantifier  for the lazy repeat , pattern   matches   but not.

As a practical application of lazy quantifiers, consider matching C/C++ multiline comments of the form. The lexer specification pattern  with lazy repeat   matches multiline comments. Without lazy repeats the pattern  should be used (note that quotation of the form   is allowed in lexer specifications only, this construct is comparable to the   quotations supported by most regex libraries.)

Other pattern matchers
Besides the built-in RE/flex POSIX regex pattern matcher, RE/flex also supports PCRE2, Boost.Regex and std::regex pattern matching libraries. PCRE2 and Boost.Regex offer a richer regular expression pattern syntax with Perl pattern matching semantics, but are slower due to their intrinsic NFA-based matching algorithm.

Translation
Lex, Flex and RE/flex translate regular expressions to DFA, which are implemented in tables for run-time scanning. RE/flex differs from Lex and Flex in that the generated tables contain a list of opcode words executed by a virtual machine to perform pattern matching. In addition, a DFA implemented in code instead of opcode tables is generated with the  option.

For example, the following direct-coded DFA for pattern  is generated with option  :

A list of virtual machine opcode words for pattern  is generated with option  :

Debugging and profiling
The RE/flex built-in profiler can be used to measure the performance of the generated scanner automatically. The profiler instruments the scanner source code to collect run-time metrics. At run-time when the instrumented scanner terminates, the profiler reports the number of times a rule is matched and the cumulative time consumed by the matching rule. Profiling includes the time spent in the parser when the rule returns control to the parser. This allows for fine-tuning the performance of the generated scanners and parsers. Lexer rules that are hot spots, i.e. computationally expensive, are detected and can be optimized by the user in the lexer source code.

Also debugging of the generated scanner is supported with Flex-compatible options. Debugging outputs annotated lexer rules during scanning.