f1131623_sion_Reference__Special_Groups__Description
[Feature] Named conditional [Syntax] (?(name)then|else) where name is the name of a capturing group and then and else are any valid regexes [Example] (?a)?(?(one)b|c) matches ab, the first c, and the second c in babxcac [JGsoft] YES [.NET] YES [Java] no [Perl] no [PCRE] 6.7 [PCRE2] YES [PHP] 5.2.0 [Delphi] YES [R] YES [JavaScript] no [VBScript] no [XRegExp] no [Python] YES [Ruby] no [std::regex] no [Tcl ARE] no [POSIX BRE] no [POSIX ERE] no [GNU BRE] no [GNU ERE] no [Oracle] no [XML] no [XPath] no [Description] 
If the capturing group with the given name took part in the match attempt thus far, the "then" part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the "else" part must match for the overall regex to match.
Regular Expression Reference: Special Groups