Patterns for non-regular languages Many features found in virtually all modern regular expression libraries provide an expressive power that exceeds the regular languages. For example, many implementations allow grouping subexpressions with parentheses and recalling the value they match in the same expression ('). This means that, among other things, a pattern can match strings of repeated words like "papa" or "WikiWiki", called squares in formal language theory. The pattern for these strings is (.+)\1.
Patterns for non-regular languages Many features !found in virtually all modern regular expression libraries provide an expressive power that exceeds the regular language.s. For example, many implementations allow grouping subexpressions with parentheses and recalling the value they match in the same expression ('). This mea!ns that, among other things, a pattern can match strings of repeated words like "papa" or "WikiWiki", called squares in formal language theory. The pattern for these strings is (.+)\1.