Backtracking Control Verb Argument

Perl and PCRE2 provide away for the replacement string to insert the argument of the last backtracking control verb that participated in the match.

PCRE2 supports $*MARK and ${*MARK} as part of its replacement text syntax. Replacing a+(*:LEFT)|c+(*:RIGHT) with $*MARK in abc yields LEFTbRIGHT in PCRE2. The curly braces are needed if you want to include some letters and digits immediately after the argument in the replacement string. If the regex does not have any backtracking control verbs with arguments or none of them participated in the match attempt then $*MARK and ${*MARK} are replaced with the empty string.

Perl stores the argument of the last control verb that was involved in the match in the variable $REGMARK. In a Perl script you can use this variable in the replacement because Perl interpolates variables in replacement strings. Replacing a+(*:LEFT)|c+(*:RIGHT) with $REGMARK in abc yields LEFTbRIGHT in a Perl script. If the regex does not have any backtracking control verbs with arguments or none of them participated in the match attempt then $REGMARK is set to TRUE which is interpolated as 1. So replacing [ab] with $REGMARK in abc yields 1b1.