QRegExp: Add a means to do lookbacks in a regexp

QRegExp: Add a means to do lookbacks in a regexp

[Description from duplicate task QTBUG-11444]:
Positive and Negative Lookbehind allow one to match an expression that is (or is not) preceded by another expression. See http://www.regular-expressions.info/lookaround.html for details. This is a serious limitation, since lookbehind constraints for more than one character cannot be expressed any other way, and since QRegExp currently does not support Positive and Negative Lookbehind. Relevant discussion can be found here: http://stackoverflow.com/questions/1213525/regexp-to-match-string1-unless-preceded-by-string2