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 of substitution
eval "$regex";