+* ? in regexp: Regexps.
+* ?? in regexp: Regexps.
+* [ in regexp: Regexps.
+* \ in regexp: Regexps.
+* \' in regexp: Regexps.
+* \(?: in regexp: Regexps.
+* \< in regexp: Regexps.
+* \= in regexp: Regexps.
+* \> in regexp: Regexps.
+* \` in regexp: Regexps.
+* \B in regexp: Regexps.
+* \b in regexp: Regexps.
+* \S in regexp: Regexps.
+* \s in regexp: Regexps.
+* \W in regexp: Regexps.
+* \w in regexp: Regexps.
+* \{n,m\} in regexp: Regexps.
+* ] in regexp: Regexps.
+* ^ in regexp: Regexps.