If you expect to be using regular expressions frequently, it might be worth your while to write a classmethod that extends Ens.Rule.FunctionSet and wraps $MATCH or $LOCATE in something that can be called from a business rule (or DTL for that matter).
/// Custom methods for business rules and DTLs
Class User.Rule.FunctionSet Extends Ens.Rule.FunctionSet
/// Accepts a string <var>pString</var> and regular expression pattern <var>pPattern</var>
/// as arguments; returns 0 for no match, and a positive integer indicating the match's
/// position if there is a match.
ClassMethod REMatch(pString As %String, pPattern As %String) As %Integer
You could use $MATCH instead of $LOCATE in the method above, but $MATCH() assumes the supplied pattern is begin- and end-anchored. In other words, $LOCATE("this","is") returns the positive integer 3, which for all Cache boolean purposes evaluates as true. $MATCH("this","is") returns 0 (false) since "^is$" does not match "this".
If the class you create extends Ens.Rule.FunctionSet and resides in your production's namespace, it will be selectable from the function dropdown list in both the Business Rule and DTL expression editors.