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)) {}

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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...

In reply to how can I combine these expressions? by knight

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