$ python -c 'import regex; print(regex.sub("^(?<a>.)|(?<c>.)$|(?<b>.)", lambda m:m.lastgroup, "xxx"))'
abc

##</code><code>##

$ perl -E'say "xxx" =~ s/^(?<a>.)|(?<c>.)$|(?<b>.)/@{[keys %+]}/gr'
abc