[ruby-core:90666] [Ruby trunk Bug#15450] Add a method `String#each_match` to the Ruby core

Issue #15450 has been reported by CaryInVictoria (Cary Swoveland).

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Bug #15450: Add a method `String#each_match` to the Ruby core
https://bugs.ruby-lang.org/issues/15450

* Author: CaryInVictoria (Cary Swoveland)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN
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`String#each_match` would have two forms:

*each_match(pattern) { |match| block } → str*
*each_match(pattern) → an_enumerator*

The latter would be identical to the form *gsub(pattern) → enumerator* of [String#gsub](http://ruby-doc.org/core-2.5.1/String.html#method-i-gsub). The former would simply yield the matches to a block and return the receiver.

I frequently use the form of `gsub` that returns an enumerator instead of `scan` when chaining to Enumerable methods. That's because `scan` returns an unneeded temporary array. This use of `gsub` can also be useful when the pattern contains capture groups, which can be a complication when using `scan`, as in the following example

Suppose we are given a string and wish to count the number of occurrences of each word that begins and ends with the same letter (case-insensitive).

     str = "Viv and Bob are party animals. Bob and Eve are a couple who met on Christmas Eve. Bob is a regular guy."

     r = /\b(?:[a-z]|([a-z])[a-z]*\1)\b/i

This regular expression reads, "match a word break, followed by one letter or by two or more letters with the last matching the first (case insensitive), all followed by a word break".

     enum = str.each_match(r)
        #=> #<Enumerator: "Viv and Bob are party...a regular guy.":gsub(/\b(?:[a-z]|([a-z])[a-z]*\1)\b/i)> 
 
We can convert `enum` to an array to see the words that will be generated by the enumerator and passed to the block.

    enum.to_a
        #=> ["Viv", "Bob", "Bob", "Eve", "a", "Eve", "Bob", "a", "regular"] 

Continuing, 

    enum.each_with_object(Hash.new(0)) { |word, h| h[word] += 1 }
       #=> {"Viv"=>1, "Bob"=>3, "Eve"=>2, "a"=>2, "regular"=>1} 

We could alternatively use `each_match` with a block.

     h = Hash.new(0)
     str.each_match(r) { |word| h[word] += 1 }
        #=> "Viv and Bob are party animals. Bob and Eve are a couple who met on Christmas Eve. Bob is a regular guy."
     h #=> {"Viv"=>1, "Bob"=>3, "Eve"=>2, "a"=>2, "regular"=>1} 

This form of `each_match` has no counterpart with `gsub`.

Consider now how `scan` would be used here. Because of the way `scan` treats capture groups, we cannot write

    str.scan(r)
       #=> [["V"], ["B"], ["B"], ["E"], [nil], ["E"], ["B"], [nil], ["r"]] 

Instead we must add a second capture group.

    arr = str.scan(/\b((?:[a-z]|([a-z])[a-z]*\2))\b/i)
       #=> [["Viv", "V"], ["Bob", "B"], ["Bob", "B"], ["Eve", "E"], ["a", nil], ["Eve", "E"], ["Bob", "B"], ["a", nil], ["regular", "r"]]

Then

    arr.each_with_object(Hash.new(0)) { |(word,_),h| h[word] += 1 }
       #=> {"Viv"=>1, "Bob"=>3, "Eve"=>2, "a"=>2, "regular"=>1}

This works but it's a bit of a [dog's breakfast](https://dictionary.cambridge.org/us/dictionary/english/a-dog-s-breakfast) when compared to the use of the proposed method.

The problem with using `gsub` in this way is that it is confusing to readers who are expecting character substitutions to be performed. I also believe that the name of this method (the "sub" in `gsub`) has resulted in the form of the method that returns an enumerator to be under-appreciated and under-used.

Some comments below propose that, in time, the form of `gsub` that returns an enumerator be deprecated.


  



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