# regexp-examples
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Extends the Regexp class with the method: Regexp#examples

This method generates a list of (some\*) strings that will match the given regular expression

\* 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](#configuration-options).

## Usage

```ruby
/a*/.examples #=> ['', 'a', 'aa']
/ab+/.examples #=> ['ab', 'abb', 'abbb']
/this|is|awesome/.examples #=> ['this', 'is', 'awesome']
/https?:\/\/(www\.)?github\.com/.examples #=> ['http://github.com',
  # 'http://www.github.com', 'https://github.com', 'https://www.github.com']
/(I(N(C(E(P(T(I(O(N)))))))))*/.examples #=> ["", "INCEPTION", "INCEPTIONINCEPTION"]
/\x74\x68\x69\x73/.examples #=> ["this"]
/\u6829/.examples #=> ["栩"]
/what about (backreferences\?) \1/.examples
  #=> ['what about backreferences? backreferences?']
```

## Installation

Add this line to your application's Gemfile:

```ruby
gem 'regexp-examples'
```

And then execute:

    $ bundle

Or install it yourself as:

    $ gem install regexp-examples

## Supported syntax

* 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>/`
  * Groups work fine, 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")
* **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...
* Some named properties, e.g. `/\p{Arabic}/`, list non-matching examples for ruby 2.0/2.1 (as the definitions changed in ruby 2.2). This will be fixed in version 1.1.0 (see the pending pull request)!

There are also some various (increasingly obscure) unsupported bits of syntax; some of which I haven't yet investigated. Much of this is not even mentioned in the ruby docs! Full documentation on all the intricate obscurities in the ruby (version 2.x) regexp parser can be found [here](https://raw.githubusercontent.com/k-takata/Onigmo/master/doc/RE). To name a few:
* Conditional capture groups, e.g. `/(group1)? (?(1)yes|no)/.examples` (which *should* return: `["group1 yes", " no"]`)
* Back reference by relative group number, e.g. `/(a)(b)(c)(d) \k<-2>/.examples` (which *should* return: `["abcd c"]`)
* Back reference using single quotes, and for group numbers, e.g. `/(a) \k'1'/.examples` (which is really just alternative syntax for `/(a) \1/`!)

## 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, there are many good blog posts out on the internet. The [wikipedia entry](http://en.wikipedia.org/wiki/Regular_expression)'s not bad either.

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

* Lookarounds, e.g. `/foo(?=bar)/`, `/foo(?!bar)/`, `/(?<=foo)bar/`, `/(?<!foo)bar/`
* [Anchors](http://ruby-doc.org/core-2.2.0/Regexp.html#class-Regexp-label-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]`

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

```ruby
/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']
```

_**WARNING**: Choosing huge numbers, along with a "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 _trillion_) examples!

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

A more sensible use case might be, for example, to generate one random 1-4 digit string:

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

(Note: I may develop a much more efficient way to "generate one example" in a later release of this gem.)

## TODO

* Performance improvements:
  * Use of lambdas/something (in [constants.rb](lib/regexp-examples/constants.rb)) to improve the library load time. See the pending pull request.
  * (Maybe?) add a `max_examples` configuration option and use lazy evaluation, to ensure the method never "freezes".
* Potential future feature: `Regexp#random_example` - but implementing this properly is non-trivial, due to performance issues that need addressing first!
* Write a blog post about how this amazing gem works! :)

## Contributing

1. Fork it ( https://github.com/[my-github-username]/regexp-examples/fork )
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!!