Regexp::Parser Gem Version Build Status Code Climate

A ruby library to help with lexing, parsing, and transforming regular expressions.

  • Multilayered
    • A scanner based on ragel
    • A lexer that produces a “stream” of tokens
    • A parser that produces a “tree” of Regexp::Expression objects (OO API)
  • Supports ruby 1.8, 1.9, and all but one of the 2.x expressions See Scanner Syntax
  • Supports ruby 1.8, 1.9, 2.0, and 2.1 runtimes.

For an example of regexp_parser in use, see the meta_re project

Requirements

  • ruby ‘1.8.7’..‘2.1.3’
  • ragel, but only if you want to build the gem or work on the scanner

Note: See the .travis.yml file for covered versions.

Install

gem install regexp_parser

Usage

# require the gem, then call one of:
require 'regexp_parser'

# The Scanner
Regexp::Scanner.scan regexp

# The Lexer
Regexp::Lexer.scan regexp

# Or the Parser
Regexp::Parser.parse regexp

All three can either return their results or take a block to perform further handling.

Components

Scanner

A ragel generated scanner that recognizes the cumulative syntax of both
supported flavors. Breaks the expression’s text into tokens, including
their type, token, text, and start/end offsets within the original
pattern.

Example

The following scans the given pattern and prints out the type, token, text and
start/end offsets for each token found.

require 'regexp_parser'

Regexp::Scanner.scan /(ab?(cd)*[e-h]+)/  do |type, token, text, ts, te|
  puts "type: #{type}, token: #{token}, text: '#{text}' [#{ts}..#{te}]"
end

# output
# type: group, token: capture, text: '(' [0..1]
# type: literal, token: literal, text: 'ab' [1..3]
# type: quantifier, token: zero_or_one, text: '?' [3..4]
# type: group, token: capture, text: '(' [4..5]
# type: literal, token: literal, text: 'cd' [5..7]
# type: group, token: close, text: ')' [7..8]
# type: quantifier, token: zero_or_more, text: '*' [8..9]
# type: set, token: open, text: '[' [9..10]
# type: set, token: range, text: 'e-h' [10..13]
# type: set, token: close, text: ']' [13..14]
# type: quantifier, token: one_or_more, text: '+' [14..15]
# type: group, token: close, text: ')' [15..16]

A one-liner that returns an array of the textual parts of the given pattern:

Regexp::Scanner.scan( /(cat?([bhm]at)){3,5}/ ).map {|token| token[2]}
#=> ["(", "cat", "?", "(", "[", "b", "h", "m", "]", "at", ")", ")", "{3,5}"]

Notes

  • The scanner performs basic syntax error checking, like detecting missing
    balancing punctuation and premature end of pattern. Flavor validity checks
    are performed in the lexer.

  • If the input is a ruby Regexp object, the scanner calls #source on it to
    get its string representation. #source does not include the options of
    expression (m, i, and x) To include the options the scan, #to_s should
    be called on the Regexp before passing it to the scanner, or any of the
    higher layers.

  • To keep the scanner simple® and fairly reusable for other purposes, it
    does not perform lexical analysis on the tokens, sticking to the task
    of tokenizing and leaving lexical analysis upto to the lexer.

Syntax

Defines the supported tokens for a specific engine implementation (aka a
flavor). Syntax classes act as lookup tables, and are layered to create
flavor variations. Syntax only comes into play in the lexer.

Example

The following instantiates the syntax for Ruby 1.9 and checks a couple of its
implementations features, and then does the same for Ruby 1.8:

require 'regexp_parser'

ruby_19 = Regexp::Syntax.new 'ruby/1.9'
ruby_19.implements? :quantifier, :zero_or_one             # => true
ruby_19.implements? :quantifier, :zero_or_one_reluctant   # => true
ruby_19.implements? :quantifier, :zero_or_one_possessive  # => true

ruby_18 = Regexp::Syntax.new 'ruby/1.8'
ruby_18.implements? :quantifier, :zero_or_one             # => true
ruby_18.implements? :quantifier, :zero_or_one_reluctant   # => true
ruby_18.implements? :quantifier, :zero_or_one_possessive  # => false

Notes

  • Variatiions on a token, for example a named group with < and > vs one with a pair of single quotes, are specified with an underscore followed by two characters appended to the base token. In the previous named group example, the tokens would be :named_ab (angle brackets) and :named_sq (single quotes). These variations are normalized by the syntax to :named.

Lexer

Sits on top of the scanner and performs lexical analysis on the tokens that
it emits. Among its tasks are breaking quantified literal runs, collecting the
emitted token structures into an array of Token objects, calculating their
nesting depth, normalizing tokens for the parser, and checkng if the tokens
are implemented by the given syntax flavor.

Tokens are Struct objects, with a few helper methods; #next, #previous, #offsets
and #length.

Example

The following example scans the given pattern, checks it against the ruby 1.8
syntax, and prints the token objects’ text.

require 'regexp_parser'

Regexp::Lexer.scan /a?(b(c))*[d]+/ do |token|
  puts "#{'  ' * token.level}#{token.text}"
end

# output
# a
# ?
# (
#   b
#   (
#     c
#   )
# )
# *
# [
# d
# ]
# +

A one-liner that returns an array of the textual parts of the given pattern.
Compare the output with that of the one-liner example of the Scanner; notably
how the sequence ‘cat’ is treated.

Regexp::Lexer.scan( /(cat?([b]at)){3,5}/ ).map {|token| token.text}
#=&gt; ["(", "ca", "t", "?", "(", "[", "b", "]", "at", ")", ")", "{3,5}"]

Notes

  • The default syntax is that of the latest released version of ruby.

  • The lexer performs some basic parsing to determine the depth of the
    emitted tokens. This responsibility might be relegated to the scanner
    in a future release.

Parser

Sits on top of the lexer and transforms the “stream” of Token objects emitted
by it into a tree of Expression objects represented by an instance of the
Expression::Root class. See Expression below for more information.

Example

require 'regexp_parser'

regex = /a?(b)*[c]+/m

# using #to_s on the Regexp object to include options. Note that this turns the
# expression into '(?m-ix:a?(b)*[c]+)', thus the Group::Options in the output
root = Regexp::Parser.parse( regex.to_s, 'ruby/2.1')

root.multiline?         # =&gt; true (aliased as m?)
root.case_insensitive?  # =&gt; false (aliased as i?)

# simple tree walking method (depth-first, pre-order)
def walk(e, depth = 0)
  puts "#{'  ' * depth}&gt; #{e.class}"

  if e.respond_to?(:expressions)
    e.each {|s| walk(s, depth+1) }
  end
end

walk(root)

# output
# &gt; Regexp::Expression::Root
#   &gt; Regexp::Expression::Group::Options
#     &gt; Regexp::Expression::Literal
#     &gt; Regexp::Expression::Group::Capture
#       &gt; Regexp::Expression::Literal
#     &gt; Regexp::Expression::CharacterSet

Note: quantifiers do not appear in the output because they are members of the
Expression class. See the next section for details.

Expression

The base class of all objects returned by the parser, implements most of the
functions that are common to all expression classes.

Each Expression object contains the following members:

  • quantifier: an instance of Expression::Quantifier that holds the details of repetition for the Expression. Has a nil value if the expression is not quantified.
  • expressions: an array, holds the sub-expressions for the expression if it is a group or alternation expression. Empty if the expression doesn’t have sub-expressions.
  • options: a hash, holds the keys :i, :m, and :x with a boolean value that indicates if the expression has a given option.

Expressions also contain the following members from the scanner/lexer:

  • type: a symbol, denoting the expression type, such as :group, :quantifier
  • token: a symbol, for the object’s token, or opening token (in the case of groups and sets)
  • text: a string, the text of the expression (same as token for nesting expressions)

Every expression also has the following methods:

  • to_s: returns the string representation of the expression.
  • <<: adds sub-expresions to the expression.
  • each: iterates over the expressions sub-expressions, if any.
  • []: access sub-expressions by index.
  • quantified?: return true if the expression was followed by a quantifier.
  • quantity: returns an array of the expression’s min and max repetitions.
  • greedy?: returns true if the expression’s quantifier is greedy.
  • reluctant? or lazy?: returns true if the expression’s quantifier is reluctant.
  • possessive?: returns true if the expression’s quantifier is possessive.
  • multiline? or m?: returns true if the expression has the m option
  • case_insensitive? or ignore_case? or i?: returns true if the expression has the i option
  • free_spacing? or extended? or x?: returns true if the expression has the x option

A special expression class Expression::Sequence is used to hold the
expressions of a branch within an Expression::Alternation expression. For
example, the expression ‘bat|cat|hat’ would result in an alternation with 3
sequences, one for each possible alternative.

Scanner Syntax

The following syntax elements are supported by the scanner.

  • Alternation: a|b|c, etc.
  • Anchors: , $, \b, etc.
  • Character Classes (aka Sets): [abc], []]
  • Character Types: \d, \H, \s, etc.
  • Escape Sequences: \t, +, \?, etc.
  • Grouped Expressions
    • Assertions
    • Lookahead: (?=abc)
    • Negative Lookahead: (?!abc)
    • Lookabehind: (?<=abc)
    • Negative Lookbehind: (?<!abc)
    • Atomic: (?>abc)
    • Back-references:
    • Named: \k
    • Nest Level: \k
    • Numbered: \k<1>
    • Relative: \k<-2>
    • Capturing: (abc)
    • Comment: (?# comment)
    • Named: (?abc)
    • Options: (?mi-x:abc)
    • Passive: (?:abc)
    • Sub-expression Calls: \g, \g<1>
  • Literals: abc, def?, etc.
  • POSIX classes: [:alpha:], [:print:], etc.
  • Quantifiers
    • Greedy: ?, *, +, {m,M}
    • Reluctant: ??, *?, +?, {m,M}?
    • Possessive: ?+, *+, ++, {m,M}+
  • String Escapes
    • Control: \C-C, \cD, etc.
    • Hex: \x20, \x{701230}, etc.
    • Meta: \M-c, \M-\C-C etc.
    • Octal: \0, \01, \012
    • Unicode: \uHHHH, \u{H+ H+}
  • Traditional Back-references: \1 thru \9
  • Unicode Properties:
    • Age: \p{Age=2.1}, \P{age=5.2}, etc.
    • Classes: \p{Alpha}, \P{Space}, etc.
    • Derived Properties: \p{Math}, \P{Lowercase}, etc.
    • General Categories: \p{Lu}, \P{Cs}, etc.
    • Scripts: \p{Arabic}, \P{Hiragana}, etc.
    • Simple Properties: \p{Dash}, \p{Extender}, etc.

Missing Features

The following were added by the Onigmo regular expression library used by
ruby 2.x and are not currently recognized by the scanner:

  • Planned for support
    • Conditional Expressions: (?(cond)yes-subexp), (?(cond)yes-subexp|no-subexp)
    • Negative POSIX Brackets: [:alpha:], [:digit:]
    • New Character Set Options: d, a, and u see
  • Not planned for support
    • Keep: \K (not enabled for ruby syntax)
    • Quotes: \Q…\E (perl and java syntax only) see
    • Capture History: (?@…), (?@…) (not enabled for ruby syntax) see

See something else missing? Please submit an issue

Note: Attempting to process expressions with any of the missing syntax features will
cause an error.

Testing

To run the tests simply run rake from the root directory, as ‘test’ is the default task.

In addition to the main test task, which runs all tests, there are also component specific test
tasks, which only run the tests for one component at a time. These are:

  • test:scanner
  • test:lexer
  • test:parser
  • test:expression
  • test:syntax

A special task ‘test:full’ generatees the scanner’s code from the ragel source files and
runs all the tests. This requires ragel to be installed.

The tests use ruby’s test_unit, so they can also be run with:

ruby test/test_all.rb

This is useful when there is a need to focus on specific test files, for example:

ruby test/scanner/test_properties.rb

Building

Building the scanner and the gem requires ragel to be
installed. The build tasks will automatically invoke the ‘ragel:rb’ task to generate the
ruby scanner code.

The project uses the standard rubygems package tasks:

To build, run:

rake build

To install, run:

rake install

References

Documentation and books used while working on this project.

Ruby Flavors

  • Oniguruma Regular Expressions link
  • Read Ruby > Regexps link

Regular Expressions

  • Mastering Regular Expressions, By Jeffrey E.F. Friedl (2nd Edition) book
  • Regular Expression Flavor Comparison link
  • Enumerating the strings of regular languages link

Unicode

  • Unicode Explained, By Jukka K. Korpela. book
  • Unicode Derived Properties link
  • Unicode Property Aliases link
  • Unicode Regular Expressions link
  • Unicode Standard Annex #44 link

Thanks

This work is based on and inspired by the hard work and ideas of many people,
directly or indirectly. The following are only a few of those that should be
thanked.

  • Adrian Thurston, for developing ragel.
  • Caleb Clausen, for feedback, which inspired this, valuable insights on structuring the parser, and lots of cool code.
  • Jan Goyvaerts, for his excellent resource on regular expressions.
  • Run Paint Run Run, for his work on Read Ruby
  • Yukihiro Matsumoto, of course! For “The Ruby”, of course!
Copyright

Copyright © 2010-2014 Ammar Ali. See LICENSE file for details.