my $regex = 's/(\w+)\s+tacos/$1 yummy tacos/g';
$regex =~ s/(\\s\+)(\w)/$1(<\\w\\w>)$2/g;       # Handle first part of
+ substitution
my $match = $2;
$regex =~ s/ $match/\$2$match/g;                # Handle second part o
+f substitution
eval "$regex";
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That would handle the transform from the first to the second. Ideally, you would re-evaluate your regexes and build them using some regex builder. The builder would handle the optional tags and making sure they stayed in after the substitutions.