A minimal representation and break-down of the logical selectors regular expressions in GROUPS: logicalsel: '(is|where|matches|not|has)(?:\\x28\\s?(\\[([^\\[\\]]*)\\]|[^()\\[\\]]*|.*)\\s?\\x29)' in PATTERNS: logicalsel: RegExp('^:(?:' + GROUPS.logicalsel + ')(.*)', 'i') FINAL matcher regular expression in compiled form, double escapes removed and underscores used as separators: (1) (2) (3) (4) ----------------------------------------- ------------------------------ --------- ---- | | | | | | | | logicalsel: /____^:(?:(is|where|matches|not|has)(?:\x28\s?________(\[([^\[\]]*)\]|[^()\[\]]*|.*)________\s?\x29))________(.*)____/i (1) matches any logical pseudo-class then an open parenthesis and possible whitespace following, then (2) expression made of multiple compounds selectors surrounded by round or square brackets, then (3) possible whitespace followed by closed parenthesis, then (4) next selector group