Well, in an effort to better understand Perl's regex code, I've started writing my own. It'll be much simpler, I know, but I hope to get an idea as to how such things work -- specfically, ways to optimize some regexes. (I've been telling myself that Perl's engine is so hideous, this exercise will end up being futile. ;) But I'm doing it anyway.)

So today, I came across a scary fact. I was telling someone to use two regexes for removing leading/trailing whitespace, because one regex is a slow evil thing. Here were attempts made:

s/^\s+(.*?)\s+$/$1/;   # fails sometimes
s/^\s*(.*?)\s*$/$1/;   # succeeds, but slow
s/^\s*(.*\S)\s*$/$1/;  # fails sometimes
s/^\s*//, s/\s*$//;    # succeeds, but WHY use * ?
s/^\s+//, s/\s+$//;    # succeeds, but is it good?
[download]

That last example, I said, was great, because (I thought) Perl optimizes things like /\s+$/. I was (shockingly) wrong. It turns out that /\s$/ is optimized, but not the + version.

Eww...

For interested parties, this is how Perl executes s/\s+$// on a string like "a b  c   d    ".

$_ = "a b  c   d    ";  # 1, 2, 3, 4 spaces
s/\s+$//;

=pod

A = \s+
B = $
X = fail

"a b  c   d    "
  AX
    AAX
       AAAX
           AAAA
[download]

See? It tests at all the whitespace. How ugly. Simple regexes like that should be optimized.

So I tried the sexeger approach:

($_ = reverse) =~ s/^\s+//; $_ = reverse;
[download]

That ran faster. Then I thought about the optimization of /\s$/, and tried:

1 while s/\s$//;
[download]

That was reasonably fast too -- much faster than the s/\s+$// approach, at least.

Note that these findings are important, insofaras embedded whitespace in the string -- that's the root of the problem for the s/\s+$// approach.

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

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