comment on

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

In reply to Re^2: Inefficient regex (death to dot star) by tye
in thread Inefficient regex by Wibble

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