How to match all occurrences of a regular expression in Ruby

Question

Is there a quick way to find every match of a regular expression in Ruby? I've looked through the Regex object in the Ruby STL and searched on Google to no avail.

Answer 1

Using scan should do the trick:

string.scan(/regex/)

Answer 2

To find all the matching strings, use String's scan method.

str = "A 54mpl3 string w1th 7 numb3rs scatter36 ar0und"
str.scan(/\d+/)
#=> ["54", "3", "1", "7", "3", "36", "0"]

If you want, MatchData, which is the type of the object returned by the Regexp match method, use:

str.to_enum(:scan, /\d+/).map { Regexp.last_match }
#=> [#<MatchData "54">, #<MatchData "3">, #<MatchData "1">, #<MatchData "7">, #<MatchData "3">, #<MatchData "36">, #<MatchData "0">]

The benefit of using MatchData is that you can use methods like offset:

match_datas = str.to_enum(:scan, /\d+/).map { Regexp.last_match }
match_datas[0].offset(0)
#=> [2, 4]
match_datas[1].offset(0)
#=> [7, 8]

See these questions if you'd like to know more:

• "How do I get the match data for all occurrences of a Ruby regular expression in a string?"
• "Ruby regular expression matching enumerator with named capture support"
• "How to find out the starting point for each match in ruby"

Reading about special variables $&, $', $1, $2 in Ruby will be helpful too.

Answer 3

if you have a regexp with groups:

str="A 54mpl3 string w1th 7 numbers scatter3r ar0und"
re=/(\d+)[m-t]/

you can use String's scan method to find matching groups:

str.scan re
#> [["54"], ["1"], ["3"]]

To find the matching pattern:

str.to_enum(:scan,re).map {$&}
#> ["54m", "1t", "3r"]

Or the solution to have the complete matchdata:

str.to_enum(:scan,re).map{Regexp.last_match}
#> [#<MatchData "54m" 1:"54">, #<MatchData "1t" 1:"1">, #<MatchData "3r" 1:"3">]

str.to_enum(:scan,re).map {$~}
#> [#<MatchData "54m" 1:"54">, #<MatchData "1t" 1:"1">, #<MatchData "3r" 1:"3">]

Answer 4

You can use string.scan(your_regex).flatten. If your regex contains groups, it will return in a single plain array.

string = "A 54mpl3 string w1th 7 numbers scatter3r ar0und"
your_regex = /(\d+)[m-t]/
string.scan(your_regex).flatten
=> ["54", "1", "3"]

Regex can be a named group as well.

string = 'group_photo.jpg'
regex = /\A(?<name>.*)\.(?<ext>.*)\z/
string.scan(regex).flatten

You can also use gsub, it's just one more way if you want MatchData.

str.gsub(/\d/).map{ Regexp.last_match }

Answer 5

If you have capture groups () inside the regex for other purposes, the proposed solutions with String#scan and String#match are problematic:

1. String#scan only get what is inside the capture groups;
2. String#match only get the first match, rejecting all the others;
3. String#matches (proposed function) get all the matches.

On this case, we need a solution to match the regex without considering the capture groups.

String#matches

With the Refinements you can monkey patch the String class, implement the String#matches and this method will be available inside the scope of the class that is using the refinement. It is an incredible way to Monkey Patch classes on Ruby.

Setup

• /lib/refinements/string_matches.rb

# This module add a String refinement to enable multiple String#match()s
# 1. `String#scan` only get what is inside the capture groups (inside the parens)
# 2. `String#match` only get the first match
# 3. `String#matches` (proposed function) get all the matches
module StringMatches
  refine String do
    def matches(regex)
      scan(/(?<matching>#{regex})/).flatten
    end
  end
end

Used: named capture groups

Usage

• rails c

> require 'refinements/string_matches'

> using StringMatches

> 'function(1, 2, 3) + function(4, 5, 6)'.matches(/function\((\d), (\d), (\d)\)/)
=> ["function(1, 2, 3)", "function(4, 5, 6)"]

> 'function(1, 2, 3) + function(4, 5, 6)'.scan(/function\((\d), (\d), (\d)\)/)
=> [["1", "2", "3"], ["4", "5", "6"]]

> 'function(1, 2, 3) + function(4, 5, 6)'.match(/function\((\d), (\d), (\d)\)/)[0]
=> "function(1, 2, 3)"

Answer 6

Return an array of MatchData objects

#scan is very limited--only returns a simple array of strings!

Far more powerful/flexible for us to get an array of MatchData objects.

I'll provide two approaches (using same logic), one using a PORO and one using a monkey patch:

PORO:

class MatchAll
  def initialize(string, pattern)
    raise ArgumentError, 'must pass a String' unless string.is_a?(String)

    raise ArgumentError, 'must pass a Regexp pattern' unless pattern.is_a?(Regexp)

    @string = string
    @pattern = pattern
    @matches = []
  end

  def match_all
    recursive_match
  end

  private

  def recursive_match(prev_match = nil)
    index = prev_match.nil? ? 0 : prev_match.offset(0)[1]

    matching_item = @string.match(@pattern, index)
    return @matches unless matching_item.present?

    @matches << matching_item
    recursive_match(matching_item)
  end
end

USAGE:

test_string = 'a green frog jumped on a green lilypad'

MatchAll.new(test_string, /green/).match_all
=> [#<MatchData "green", #<MatchData "green"]

---

Monkey patch

I don't typically condone monkey-patching, but in this case:

• we're doing it the right way by "quarantining" our patch into its own module
• I prefer this approach because 'string'.match_all(/pattern/) is more intuitive (and looks a lot nicer) than MatchAll.new('string', /pattern/).match_all

module RubyCoreExtensions
  module String
    module MatchAll
      def match_all(pattern)
        raise ArgumentError, 'must pass a Regexp pattern' unless pattern.is_a?(Regexp)

        recursive_match(pattern)
      end

      private

      def recursive_match(pattern, matches = [], prev_match = nil)
        index = prev_match.nil? ? 0 : prev_match.offset(0)[1]

        matching_item = self.match(pattern, index)
        return matches unless matching_item.present?

        matches << matching_item
        recursive_match(pattern, matches, matching_item)
      end
    end
  end
end

I recommend creating a new file and putting the patch (assuming you're using Rails) there /lib/ruby_core_extensions/string/match_all.rb

To use our patch we need to make it available:

# within application.rb
require './lib/ruby_core_extensions/string/match_all.rb'

Then be sure to include it in the String class (you could put this wherever you want; but for example, right under the require statement we just wrote above. After you include it once, it will be available everywhere, even outside the class where you included it).

String.include RubyCoreExtensions::String::MatchAll

USAGE: And now when you use #match_all you get results like:

test_string = 'hello foo, what foo are you going to foo today?'

test_string.match_all /foo/
=> [#<MatchData "foo", #<MatchData "foo", #<MatchData "foo"]

test_string.match_all /hello/
=> [#<MatchData "hello"]

test_string.match_all /none/
=> []

---

I find this particularly useful when I want to match multiple occurrences, and then get useful information about each occurrence, such as which index the occurrence starts and ends (e.g. match.offset(0) => [first_index, last_index])