f1131623_sion_Reference__Special_Groups__Description
[Feature] Positive lookahead [Syntax] (?=regex) [Example] t(?=s) matches the second t in streets. [JGsoft] YES [.NET] YES [Java] YES [Perl] YES [PCRE] YES [PCRE2] YES [PHP] YES [Delphi] YES [R] YES [JavaScript] YES [VBScript] YES [XRegExp] YES [Python] YES [Ruby] YES [std::regex] ECMA [Tcl ARE] YES [POSIX BRE] no [POSIX ERE] no [GNU BRE] no [GNU ERE] no [Oracle] no [XML] no [XPath] no [Description] 
Matches at a position where the pattern inside the lookahead can be matched. Matches only the position. It does not consume any characters or expand the match. In a pattern like one(?=two)three, both two and three have to match at the position where the match of one ends.
Regular Expression Reference: Special Groups