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Extends the Regexp class with the methods: Regexp#examples and Regexp#random_example

Regexp#examples generates a list of all* strings that will match the given regular expression.

Regexp#random_example returns one, random string (from all possible strings!!) that matches the regex.

* If the regex has an infinite number of possible srings that match it, such as /a*b+c{2,}/, or a huge number of possible matches, such as /.\w/, then only a subset of these will be listed.

For more detail on this, see configuration options.

If you'd like to understand how/why this gem works, please check out my blog post about it!


/a*/.examples #=> ['', 'a', 'aa']
/ab+/.examples #=> ['ab', 'abb', 'abbb']
/this|is|awesome/.examples #=> ['this', 'is', 'awesome']
/https?:\/\/(www\.)?github\.com/.examples #=> ['',
  # '', '', '']
/(I(N(C(E(P(T(I(O(N)))))))))*/.examples #=> ["", "INCEPTION", "INCEPTIONINCEPTION"]
/\x74\x68\x69\x73/.examples #=> ["this"]
/what about (backreferences\?) \1/.examples
  #=> ['what about backreferences? backreferences?']
/x.examples #=> ["(•_•)", "( •_•)>⌐■-■ ", "(⌐■_■)"]


# Obviously, you will get different results if you try these yourself!
/\w{10}@(hotmail|gmail)\.com/.random_example #=> "[email protected]"
  #=> "ΖΆΧͷᵦμͷηϒϰΟᵝΔ΄θϔζΌψΨεκᴪΓΕπι϶ονϵΓϹᵦΟπᵡήϴϜΦϚϴϑ͵ϴΉϺ͵ϹϰϡᵠϝΤΏΨϹϊϻαώΞΰϰΑͼΈΘͽϙͽξΆΆΡΡΉΓς"
/written by tom lord/i.random_example #=> "WrITtEN bY tOM LORD"

Supported ruby versions

  • MRI 2.0.x
  • MRI 2.1.x
  • MRI 2.2.x
  • MRI 2.3.x
  • MRI 2.4.0-dev

MRI ≤ 1.9.3 are not supported. This is primarily because MRI 2.0.0 introduced a new regexp engine (Oniguruma was replaced by Onigmo). Whilst most of this gem could be made to work with MRI 1.9.x (or even 1.8.x), I feel the changes are too significant to implement backwards compatability (especially since long-term support for MRI 1.9.3 has now ended).

For example, named properties (e.g. /\p{Alpha}/) are illegal syntax on MRI 1.9.3.

Other implementations, such as JRuby, could probably work fine - but I haven't fully tried/tested it. Pull requests are welcome.


Add this line to your application's Gemfile:

gem 'regexp-examples'

And then execute:

$ bundle

Or install it yourself as:

$ gem install regexp-examples

Supported syntax

Short answer: Everything is supported, apart from "irregular" aspects of the regexp language -- see impossible features.

Long answer:

  • All forms of repeaters (quantifiers), e.g. /a*/, /a+/, /a?/, /a{1,4}/, /a{3,}/, /a{,2}/
    • Reluctant and possissive repeaters work fine, too, e.g. /a*?/, /a*+/
  • Boolean "Or" groups, e.g. /a|b|c/
  • Character sets, e.g. /[abc]/ - including:
    • Ranges, e.g./[A-Z0-9]/
    • Negation, e.g. /[^a-z]/
    • Escaped characters, e.g. /[\w\s\b]/
    • POSIX bracket expressions, e.g. /[[:alnum:]]/, /[[:^space:]]/
    • Set intersection, e.g. /[[a-h]&&[f-z]]/
  • Escaped characters, e.g. /\n/, /\w/, /\D/ (and so on...)
  • Capture groups, e.g. /(group)/
    • Including named groups, e.g. /(?<name>group)/
    • And backreferences(!!!), e.g. /(this|that) \1/ /(?<name>foo) \k<name>/
    • ...even for the more "obscure" syntax, e.g. /(?<future>the) \k'future'/, /(a)(b) \k<-1>/`
    • ...and even if nested or optional, e.g. /(even(this(works?))) \1 \2 \3/, /what about (this)? \1/
    • Non-capture groups, e.g. /(?:foo)/
    • Comment groups, e.g. /foo(?#comment)bar/
  • Control characters, e.g. /\ca/, /\cZ/, /\C-9/
  • Escape sequences, e.g. /\x42/, /\x5word/, /#{"\x80".force_encoding("ASCII-8BIT")}/
  • Unicode characters, e.g. /\u0123/, /\uabcd/, /\u{789}/
  • Octal characters, e.g. /\10/, /\177/
  • Named properties, e.g. /\p{L}/ ("Letter"), /\p{Arabic}/ ("Arabic character") , /\p{^Ll}/ ("Not a lowercase letter"), /\P{^Canadian_Aboriginal}/ ("Not not a Canadian aboriginal character")
    • ...Even between different ruby versions!! (e.g. /\p{Arabic}/.examples(max_group_results: 999) will give you a different answer in ruby v2.1.x and v2.2.x)
  • Arbitrarily complex combinations of all the above!

  • Regexp options can also be used:

    • Case insensitive examples: /cool/i.examples #=> ["cool", "cooL", "coOl", "coOL", ...]
    • Multiline examples: /./m.examples #=> ["\n", "a", "b", "c", "d"]
    • Extended form examples: /line1 #comment \n line2/x.examples #=> ["line1line2"]
    • Options toggling supported: /before(?imx-imx)after/, /before(?imx-imx:subexpr)after/

Bugs and Not-Yet-Supported syntax

  • There are some (rare) edge cases where backreferences do not work properly, e.g. /(a*)a* \1/.examples - which includes "aaaa aa". This is because each repeater is not context-aware, so the "greediness" logic is flawed. (E.g. in this case, the second a* should always evaluate to an empty string, because the previous a* was greedy!) However, patterns like this are highly unusual...

Since the Regexp language is so vast, it's quite likely I've missed something (please raise an issue if you find something)! The only missing feature that I'm currently aware of is:

  • Conditional capture groups, e.g. /(group1)? (?(1)yes|no)/.examples (which should return: ["group1 yes", " no"])

Some of the most obscure regexp features are not even mentioned in the ruby docs! However, full documentation on all the intricate obscurities in the ruby (version 2.x) regexp parser can be found here.

Impossible features ("illegal syntax")

The following features in the regex language can never be properly implemented into this gem because, put simply, they are not technically "regular"! If you'd like to understand this in more detail, check out what I had to say in my blog post about this gem.

Using any of the following will raise a RegexpExamples::IllegalSyntax exception:

  • Lookarounds, e.g. /foo(?=bar)/, /foo(?!bar)/, /(?<=foo)bar/, /(?<!foo)bar/
  • Anchors (\b, \B, \G, ^, \A, $, \z, \Z), e.g. /\bword\b/, /line1\n^line2/
    • However, a special case has been made to allow ^, \A and \G at the start of a pattern; and to allow $, \z and \Z at the end of pattern. In such cases, the characters are effectively just ignored.
  • Subexpression calls (\g), e.g. /(?<name> ... \g<name>* )/

(Note: Backreferences are not really "regular" either, but I got these to work with a bit of hackery!)

Configuration Options

When generating examples, the gem uses 2 configurable values to limit how many examples are listed:

  • max_repeater_variance (default = 2) restricts how many examples to return for each repeater. For example:

    • .* is equivalent to .{0,2}
    • .+ is equivalent to .{1,3}
    • .{2,} is equivalent to .{2,4}
    • .{,3} is equivalent to .{0,2}
    • .{3,8} is equivalent to .{3,5}
  • max_group_results (default = 5) restricts how many characters to return for each "set". For example:

    • \d is equivalent to [01234]
    • \w is equivalent to [abcde]
    • [h-s] is equivalent to [hijkl]
    • (1|2|3|4|5|6|7|8) is equivalent to [12345]

Rexexp#examples makes use of both these options; Rexexp#random_example only uses max_repeater_variance, since the other option is redundant!

To use an alternative value, simply pass the configuration option as follows:

/a*/.examples(max_repeater_variance: 5)
  #=> [''. 'a', 'aa', 'aaa', 'aaaa' 'aaaaa']
/[F-X]/.examples(max_group_results: 10)
  #=> ['F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O']
/.*/.random_example(max_repeater_variance: 50)
  #=> "A very unlikely result!"

WARNING: Choosing huge numbers for Regexp#examples and/or a sufficiently "complex" regex, could easily cause your system to freeze!

For example, if you try to generate a list of all 5-letter words: /\w{5}/.examples(max_group_results: 999), then since there are actually 63 "word" characters (upper/lower case letters, numbers and "_"), this will try to generate 63**5 #=> 992436543 (almost 1 billion) examples!

In other words, think twice before playing around with this config!

A more sensible use case might be, for example, to generate all 1-4 digit strings:

/\d{1,4}/.examples(max_repeater_variance: 3, max_group_results: 10)

Due to code optimisation, this is not something you need to worry about (much) for Regexp#random_example. For instance, the following takes no more than ~ 1 second on my machine:

/.*\w+\d{100}/.random_example(max_repeater_variance: 1000)


  • Performance improvements:
    • (Maybe?) add a max_examples configuration option and use lazy evaluation, to ensure the method never "freezes".


  1. Fork it ( )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request
  6. Don't forget to add tests!!