CODEWITHSUNDEEP
rubyregex
R.D.
R.D.

Reputation: 2631

Why does ruby return only the last match of the regular expression?

My regular expression is the following (\d+_)* and the test string is 1_2_3_. Ruby is matching the string correctly. However the matchdata only returns "3_" as the match.

e.g.

irb(main):004:0> /(\d+_)*/.match("1_2_3_")
=> #<MatchData "1_2_3_" 1:"3_">

I'd expect something like #<MatchData "1_2_3_" 1:"1_", 2:"2_", 3:"3_">

Upvotes: 2

Views: 618

Answers (3)

Tim Pietzcker
Tim Pietzcker

Reputation: 336178

Each new repetition of the group overwrites the previous match. All regex engines work that way. To my knowledge, only the .NET regex engine provides a means to access all the matches of a repeated group (a so-called "capture").

Imagine what's happening. In a regex, every pair of parentheses builds a capturing group; they are numbered from left to right. So in /(\d+_)*/, (\d+_) is capturing group number 1.

Now if you apply that regex to 1_2_, what happens?

  • (\d+_) matches 1_
  • 1_ is stored as the contents of the first capturing group. You could now access \1 to see these contents.
  • The * tells the regex engine to retry the match from the current position.
  • (\d+_) now matches 2_
  • That text, 2_, again needs to be stored in group number 1/backreference \1. So it overwrites whatever is in there.

To get the desired result in Ruby, you need to do two regex matches: /(?:\d+_)*/ for the overall match and /\d+_/ for each single match:

irb(main):001:0> s = "1_2_3_"
=> "1_2_3_"
irb(main):009:0> s.match(/(?:\d+_)*/)
=> #<MatchData "1_2_3_">
irb(main):007:0> s.scan(/\d+_/)
=> ["1_", "2_", "3_"]

Upvotes: 5

Alex.Bullard
Alex.Bullard

Reputation: 5563

"1_2_3_".scan(/\d+_/) # =>  ["1_", "2_", "3_"]

will get you what you are looking for. (notice the removal of the *). I also removed the grouping b/c it simply results in an array of arrays, i.e [["1_"], ["2_"], ["3_"]]

Upvotes: 0

Bill Criswell
Bill Criswell

Reputation: 32921

I believe you want .scan. It'll return an array of the matches.

Upvotes: 0

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