f1131623_sion_Reference__Special_Groups__Description
[Feature] Lookaround conditional [Syntax] (?(?=regex)then|else) where (?=regex) is any valid lookaround and then and else are any valid regexes [Example] (?(?<=a)b|c) matches the second b and the first c in babxcac [JGsoft] YES [.NET] YES [Java] no [Perl] YES [PCRE] YES [PCRE2] YES [PHP] YES [Delphi] YES [R] YES [JavaScript] no [VBScript] no [XRegExp] no [Python] no [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 lookaround succeeds, the "then" part must match for the overall regex to match. If the lookaround fails, the "else" part must match for the overall regex to match. The lookaround is zero-length. The "then" and "else" parts consume their matches like normal regexes.
Regular Expression Reference: Special Groups