Feature #15450
Updated by CaryInVictoria (Cary Swoveland) almost 6 years ago
`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 this suggestion be adopted and, that, in time, the form of `gsub` that returns an enumerator be deprecated.