Re^2: Inefficient regex (death to dot star)

Exactly. In more detail, what the original regex ends up doing goes like this:

<%start_(.*?)\s*(.*?)\s*%>\n?(.+?)<%end_\1%>\n?
         $1- -A- $2- -B-      $3-       C-
Quickly matches <%start_
Tries $1 matching ""
    Tries A  matching ""
        Tries $2 matching "wibble XXX"
            Tries $3 matches to just before "<%end_flub%>"
                Is forced to backtrack by C
            Tries $3 matches to just before "<%end_wibble%>"
                Is forced to backtrack by C
Tries $1 matching "w"
    Tries A  matching ""
        Tries $2 matching "ibble XXX"
            Tries $3 matches to just before "<%end_flub%>"
                Is forced to backtrack by C
            Tries $3 matches to just before "<%end_wibble%>"
                Is forced to backtrack by C
Tries $1 matching "wi"
    Tries A  matching ""
        Tries $2 matching "bble XXX"
            Tries $3 matches to just before "<%end_flub%>"
                Is forced to backtrack by C
            Tries $3 matches to just before "<%end_wibble%>"
                Is forced to backtrack by C
...
Tries $1 matching "wibble"
    Tries A  matching " "
        Tries $2 matching "XXX"
            Tries $3 matching to just before "<%end_flub%>"
                Is forced to backtrack by C
            Tries $3 matches to just before "<%end_wibble%>"
                Succeeds finding one match
[download]

which you can see wastes a lot of time.

That is one reason why Death to dot star! suggests you use character class instead of . whenever possible in a regex.

- tye

Comment on Re^2: Inefficient regex (death to dot star) Download Code

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Re: Re^2: Inefficient regex (death to dot star) by Wibble (Beadle) on Mar 05, 2003 at 21:07 UTC
This is great and very clear - thank you!! I've taken on board what you said, experimented by making LABEL long then short to prove the point and sure enough execution times vary wildy. I've altered the regex to now use (\S+) to pick up the LABEL and it now completes in sub-second time. Thanks again!!!	[reply]