Re: Finally, a $& compromise!

Replies are listed 'Best First'.
Re: Re: Finally, a $& compromise! by japhy (Canon) on Nov 28, 2001 at 07:38 UTC
Does this mean that a regexp that captures $1 implies $& which implies the performance hit for maintaining $`, $&, and $' ? Only for maintaining them for that* regex. The way that `$DIGIT` variables are supported is thus: The string being matched against is copied (via `savepvn()`) to `rx->subbeg`. The offsets of the `$DIGIT` vars are stored in the two arrays `rx->startp` and `rx->endp`. When you access `$2`, Perl does magic: It takes the beginning and ending offsets, `rx->startp[2]` and `rx->endp[2]`, and takes a substring of `rx->subbeg`. It `savepvn()`s (copies) that substring to a scalar and returns it. However, this only happens in a regex that has capturing parentheses! If you have a regex that does NOT have capturing parentheses, it does not need to copy the string. The `$DIGIT` vars are like tiny instances of `$&` that only appear when you need them. `$&` appears all the time if you use it once. Here's an example that shows that a regex that uses capturing parentheses gives you the ability to use `$&` and the like. These are three separate programs. I'm using `eval ''` so that `$&` isn't seen at the time the regexes are executed. #!/usr/bin/perl "simple" =~ /im/ and eval q{ print "<$`><$&><$'>\n" }; ### #!/usr/bin/perl "complex" =~ /.p/ and eval q{ print "<$`><$&><$'>\n" }; #<co><mp><lex> ### #!/usr/bin/perl "capture" =~ /(.t)./ and eval q{ print "<$`><$&><$'>:<$1>\n" }; #<ca><ptu><re>:<pt> [download] Does that make sense? In order to have `$1`, you have to have the string that is also used for `$&`. From 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 produce `$1`, `$2`, etc, so you also pay a price for each pattern that contains capturing parentheses. (To avoid this cost while retaining the grouping behaviour, use the extended regular expression `(?: ... )` instead.) But if you never use `$&`, $` or `$'`, 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. Some of that will be rewritten with the advent of this pragma, though. It's nice to "rewrite the books". _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker. `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re: Re: Finally, a $& compromise!
by japhy (Canon) on Nov 28, 2001 at 07:38 UTC

Does this mean that a regexp that captures $1 implies $& which implies the performance hit for maintaining $`, $&, and $' ?

for that regex

$DIGIT

The string being matched against is copied (via savepvn()) to rx->subbeg.
The offsets of the $DIGIT vars are stored in the two arrays rx->startp and rx->endp.
When you access $2, Perl does magic:
1. It takes the beginning and ending offsets, rx->startp[2] and rx->endp[2], and takes a substring of rx->subbeg.
2. It savepvn()s (copies) that substring to a scalar and returns it.

The $DIGIT vars are like tiny instances of $& that only appear when you need them. $& appears all the time if you use it once. Here's an example that shows that a regex that uses capturing parentheses gives you the ability to use $& and the like. These are three separate programs. I'm using eval '' so that $& isn't seen at the time the regexes are executed.

#!/usr/bin/perl
"simple" =~ /im/ and eval q{ print "<$`><$&><$'>\n" };

###

#!/usr/bin/perl
"complex" =~ /.p/ and eval q{ print "<$`><$&><$'>\n" };
#<co><mp><lex>

###

#!/usr/bin/perl
"capture" =~ /(.t)./ and eval q{ print "<$`><$&><$'>:<$1>\n" };
#<ca><ptu><re>:<pt>
[download]

$1

$&

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 produce $1, $2, etc, so you also pay a price for each pattern that contains 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.

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

[reply]
[d/l]
[select]