Revisit regular expression functions

As noted by @jfuerth , these functions are very useful for "just in time" harmonization.

We may want to look at:
* adding some polyfill style UDFs to pave over differences
* listing them, with suggestions about known differences

Start by reviewing the existing regular-expression functions and the differences this issue is intended to address; the payload names no files, tests, or entry points. Decide whether the deliverable is polyfill-style UDFs, a compatibility list, or both, and define the known differences that must be covered.

Written by the indexing model from the issue text.