From: sawadatsuyoshi@... Date: 2019-02-06T08:00:11+00:00 Subject: [ruby-core:91418] [Ruby trunk Feature#15446] Add a method `String#each_match` to the Ruby core Issue #15446 has been updated by sawa (Tsuyoshi Sawada). I would rather propose to have `String#scan` take an optional second argument that is comparable to the optional second argument `capture` of `String#[]` after a regexp argument: ```ruby r = /\b([a-z]|([a-z])[a-z]*\1)\b/i str[r] # => "Viv" str[r, 0] # => "Viv" str[r, 1] # => "Viv" str[r, 2] # => "V" ``` so that it should work like this: ```ruby str.scan(r) # => [["Viv", "V"], ["Bob", "B"], ["Bob", "B"], ["Eve", "E"], ["a", nil], ["Eve", "E"], ["Bob", "B"], ["a", nil], ["regular", "r"]] str.scan(r, 0) # => ["Viv", "Bob", "Bob", "Eve", "a""Eve", "Bob", "a", "regular"] str.scan(r, 1) # => ["Viv", "Bob", "Bob", "Eve", "a""Eve", "Bob", "a", "regular"] str.scan(r, 2) # => ["V", "B", "B", "E", nil, "E", "B", nil, "r"] ``` ---------------------------------------- Feature #15446: Add a method `String#each_match` to the Ruby core https://bugs.ruby-lang.org/issues/15446#change-76679 * Author: CaryInVictoria (Cary Swoveland) * Status: Open * Priority: Normal * Assignee: * Target version: ---------------------------------------- `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) #=> # 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, in time, the form of `gsub` that returns an enumerator be deprecated. -- https://bugs.ruby-lang.org/ Unsubscribe: