|Subject:||Regex Slowdown: XPath.pm:195 Uses ($`)|
On line 195 in XML::XPath within the setNodeText() function: my $parent_path = $`; This one line causes the ENTIRE perl regular expression engine to slow down dramatically. Furthermore, any libraries that simply "use XML::XPath" experience this same slowdown -- even if the the setNodeText() function is never called. This slowdown is due to the native Perl interpreter -- as stated in "perldoc perlre": WARNING: Once Perl sees that you need one of $&, $`, or $' anywhere in the program, it has to provide them for every pattern match. This may substantially slow your program. Perl uses the same mechanism to pro- duce $1, $2, etc, so you also pay a price for each pattern that con- tains capturing parentheses. (To avoid this cost while retaining the grouping behaviour, use the extended regular expression "(?: ... )" instead.) But if you never use $&, $` or $', then patterns without capturing parentheses will not be penalized. So avoid $&, $', and $` if you can, but if you can't (and some algorithms really appreciate them), once you've used them once, use them at will, because you've already paid the price. As of 5.005, $& is not so costly as the other two. If there is ANY way to alter line 195 such that $&, $`, and $' are never used, then XPath and all subsequent dependencies will obtain a huge performance boost (within the regular expression engine).