how can I combine these expressions?

knight has asked for the wisdom of the Perl Monks concerning the following question:

I'm manually interpolating values from a hash ref of internal "environment variables" into a user-supplied string. The string's magic interpolation character is '%', not '$', so "%FOO" in the string should be replaced with the value of $env->{'FOO'}, and "%{BAR}' should be replaced with $env->{'BAR'}, etc. Plus, the interpolation is recursive; if the interpolated value itself contains a '%', it gets re-expanded. Lastly, '%%' eventually gets replaced with a single '%', although this happens later, so I actually need to pass through %-pairs unchanged.

Hope I explained that clearly...

Anyway, the following code snippet does all of this the way I want:

# % expansion.  %% gets converted to % later, so expand any
# %keyword construction that doesn't have a % in front of it
# (modulo multiple %% pairs in between).
while (($str =~ s/(^|[^\%](?:\%\%)*)\%([_a-zA-Z]\w*)/"$1".$env->{$2}/g
+e) ||
       ($str =~ s/(^|[^\%](?:\%\%)*)\%\{([_a-zA-Z]\w*)\}/"$1".$env->{$
+2}/ge)) {}
[download]

But I'm doing this with two very similar expressions, the first for normal unadorned variable names ("%FOO") and the second for variable names enclosed in braces ("%{FOO}"). Can anyone suggest how these two might be combined, hopefully in a way that speeds up the processing? The obvious technique of sticking '*' after the braces in the regexes loses because I don't want to match unbalanced braces like "%FOO}".

I've been leafing through "Mastering Regular Expressions" in search of an answer, especially the section on lookahead, but that's right about where my brain starts filling up...

Comment on how can I combine these expressions? Download Code

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Re: how can I combine these expressions? by plaid (Chaplain) on Jul 27, 2000 at 03:27 UTC
How about this: `$str =~ s/(^\|[^\%](?:\%\%))\% (\{)? # Match and capture 1 or 0 braces ([_a-zA-Z]\w) (?(2)\}) # If there's anything in $2, match en +ding brace /"$1".$env->{$3}/gex; # $3 now instead of $2 here` [download] This takes advantage of perl's conditional matching operator. From perlre: `(?(condition)yes-pattern\|no-pattern) (?(condition)yes-pattern) Conditional expression. (condition) should be either an integer in parentheses (which is valid if the corresponding pair of parentheses matched), or lookahead/lookbehind/evaluate zero-width assertion. Say, m{ ( $ )? [^()]+ (?(1) $ ) }x matches a chunk of non-parentheses, possibly included in parentheses themselves.` [download]	[reply] [d/l] [select]
(Ovid) RE(2): how can I combine these expressions? by Ovid (Cardinal) on Jul 27, 2000 at 03:47 UTC
I am humbled in the presence of greatness. :) That was slick.	[reply]
RE: Re: how can I combine these expressions? by knight (Friar) on Jul 27, 2000 at 20:29 UTC
Plaid-- Stellar. That's exactly what I needed. Many thanks. (Note on some of the replies above: Simple alternation with '\|' doesn't work because it includes the braces in the selected part of the regex. Given %{FOO}, we should interpolate $env->{'FOO'}, not $env->{'{FOO}'}.)	[reply]
Re: how can I combine these expressions? by chromatic (Archbishop) on Jul 27, 2000 at 02:44 UTC
Why not just stick a ? after each brace? That means 'match zero or one times'. I don't see anything different that happens if the variable's braced, but my brain hurts today so definitely test it first. :) Update: Okay, the quick-correcting Ovid and lhoward point out the unbalanced problem. The only other thing I can think of is an alternation: `(?:{([_a-zA-Z]\w)}\|([_a-zA-Z]\w))` That part obviously goes after you've found the % sign. It will be more expensive, though. (The * operator hurts.)	[reply] [d/l]
(Ovid) RE(2): how can I combine these expressions? by Ovid (Cardinal) on Jul 27, 2000 at 03:02 UTC
chromatic: sticking a ? after each brace would still allow mismatched braces to be passed through. Since this is user supplied data, I can see a potential for a lot of typos. Cheers, Ovid	[reply]
(Ovid) Re: how can I combine these expressions? by Ovid (Cardinal) on Jul 27, 2000 at 03:20 UTC
Well, I had trouble figuring out exactly what you needed, but a rough first (untested) try was simply using alternation. I also replaced the character class with a variable to make it a bit more legible. `$valid = "(?:[_a-zA-Z]\w)"; $str =~ s/(^\|[^\%](?:\%\%))\%($valid\|\{$valid\}))/"$1".$env->{$2}/ge;` [download] If you could post some working code with sample data, I could work on something better. I tried playing with your code and couldn't really get a good feel for what you were doing. Cheers, Ovid	[reply] [d/l]
Re: how can I combine these expressions? by lhoward (Vicar) on Jul 27, 2000 at 03:03 UTC
I agree with chromatic except I see a small problem with his solution. With the regular expression: `$str =~ s/(^\|[^\%](?:\%\%))\%\{?([_a-zA-Z]\w)\}?/"$1".$env->{$2}/ge` [download] it would match on something like %{FOO or %FOO}. How to aviod that in a single RE? I'm not quite sure... Time for me to do some digging....	[reply] [d/l]