nl_tokens: Parses patterns like \ 1 \ 2 etc . either as a backreference to a group or a deciaml char code .
pl_tokens: function GroupRefOrDecChar ( text , textLoc ) { const reference = Number ( text . slice ( 1 ) ) ; if ( reference > 0 && <mask> <= capturingGroupsCount ) { return Node ( { type : 'Backreference' , kind : 'number' , number : reference , reference , } , textLoc ) ; } return Char ( text , 'decimal' , textLoc ) ; }