In Perl 5 (which is the source of most regular expression extensions beyond the basic syntax), the behavior triggered by /m in Ruby is instead triggered by the /s option (which Ruby doesn't have, though if you put one on your regex it will silently ignore it). Without the ?, the longest possible match wins: irb(main):001:0> //.match("")įinally, about the above-mentioned /m confusion (though if you want to avoid becoming confused yourself, this might be a good place to stop reading): But when something follows that section and there are captures, you get different results. If that were the whole expression, or if there were no captures, it wouldn't matter. As Ωmega says, the ? after the * means that it is a non-greedy match. In the second regular expression, (?m:\s*), the modifier has no effect at all because there are no dots in the contained expression to modify.īack to the first expression. Basically, it will match anything at all, including nothing. So your first regular expression, (?m.*?) will match any number (including zero) of any characters (including newlines). ) to match newlines, whereas normally that's the one character it doesn't match: irb(main):001:0> "a\nb" =~ /a.b/
This question is tagged Ruby, in which "multiline mode" causes the dot character (. Here's where I got confused in the original answer, because this option has different meanings in different environments. The m, in turn, enables "multiline" mode. That is, /(?m.)/ behaves the same as /./m. But you can add option letters between the ? and the :, in which case the part of the regular expression between the parentheses behaves as if you had included those option letters when creating the regular expression.
(?.) allows you to treat the part between the parentheses as a group, without affecting the set of strings captured by the matching engine. (?.) is a way of applying modifiers to the regular expression inside the parentheses.
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