Neither backtracking nor captures are really a problem needing to be fixed. Features like (?>) were added to help extend the sorts of things regular expressions can do.

To take your example, regular expressions are not the problem with why the wrong thing was matched. Your expression allowed that interpretation (or it would have with some minor changes). The problem is that you are using an expression that does not properly cover the case you claim to be looking for.

To take another example that I think shows why these features are more useful, let's match a US telephone number. A telephone number in the US can take many forms:

And that leaves out adding a 1 or 0 for long distance and extensions, which people often give as part of the number.

Matching this set of expressions requires optional characters which (if you are doing captures) requires backtracking. (Not really, but the implementation gets hairier if we discuss that part.)

So to match a phone number, we would need:

  m{ (
       (?: \( \d\d\d \) \s* ) | \d\d\d (?: -? | \s* ) ) ?
       \d\d\d (?: - | \s* ) \d\d\d\d
     )
   }x;

[download]

Obviously, this appears somewhat complicated and there is quite a bit of possibility for confusion. In this case, however, the problem is not the regex, it's the fact that the phone number format is specified fairly sloppily.

In fact, the times that I have often found the features you are questioning most useful are when I'm dealing with real world data. Because unlike the stuff (insert pompous tone) I generate, the real world is messy and inconsistent.

One of the nastiest problems I ever tried to solve was to extract tables of information from text files generated by people at various companies. You have no idea how many weird variations that people can come up with that a person can interpret, but are almost unparseable by computer. Without many of these features, we would not have gotten as far as we did.

G. Wade

In reply to Re^3: Perl regex in real life by gwadej
in thread Perl regex in real life by RezaRob

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