\2 would refer to the stuff that matched the first \d ,Īnd \4 would refer to the stuff that matches the final, \d . \1 would refer to whatever matched (\d )\s*\s*(\d ), Is done by counting left parentheses, and sub-expressions can be nested. The first (and only) set of parentheses in the expression. Whatever sequence of characters were matches the by the \w that is enclosed in Matches a line of text that begins and ends with the same word. Second parenthesized sub-expression, and so on. Sub-expression in the regular expression \2, the part that was matched by the \1 represents the part of the string that was matched by the first parenthesized ![]() Backreferences take the form \1, \2, and \3. There is one more important aspect of regular expressions on a computer: backreferences.Ī backreference is a way of referring to a substring that was matched by an earlier part of theĮxpression. (Remember that "." matches any character, " " means "one or more",Īnd the ^ and $ anchor the pattern to the beginning and end of the line) (Note: keep in mind that digits are word characters.) \b does not itself match any character but "anchors" the pattern to a word boundary. ![]() Matches foo as a complete word that is, foo must be bounded on both ends by a non-word character or by a start or end of line Matches an a at the end of a line a "$" does not match any characters itself but "anchors" the Matches an a at the beginning of a line a "^" does not match any characters itself but "anchors" the \W = any non-word character, \s = any whitespace character, \S any non-whitespace character Matches a single "word" character this is an abbreviation for other abbreviations include: Matches a string enclosed in double quotes, including the quotation marks, where the quoted string cannotĬontain any embedded double quotes the pattern ".*" would match strings with nested quotation marks, such as: Instead, they are part of the syntax that is used for Meta-characters are not part of the strings that are matched byĪ pattern. These are called meta-characters or meta-symbols. Takes on the role played by " " in the book (and to make it worse,Ĭertain characters have special purposes in regular expressions. Options in computer implementations, and the special character "|" Syntax in Section 3.2 of the CPSC 229 book. (Note in particular that the syntax is not the same as the However, most of the basics are supported by most implementations. Unfortunately, the syntax is not standardized Many textĮditors and most programming languages have some built-in support for Regular expressions are important tools for text processing. ![]() Regular expressions are patterns that can be matched against strings.
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