CodeQL library for C#
codeql/csharp-all 1.0.1 (changelog, source)

Primitive predicate string::regexpFind

Returns a substring of the receiver which matches the given regular expression. Also returns the offset within the receiver (starting at 0) at which the match occurred (occurrenceOffset), and the number of matches which occur at smaller offsets (occurrenceIndex). The regex format used is Java’s Pattern.

string regexpFind(string regexp, int occurrenceIndex, int occurrenceOffset)