For example, the following pattern string matches a target string containing exactly ten copies of capital “A”. In that case, “aa” may appear, “bb” may appear, or they may both be omitted. This expression can be made optional by placing it in a larger group and then using the ? operator. For example, the following regular-expression string matches “aa” or “bb” but not “aabb”. For example, the following string matches a minus sign or an empty string. The expr thereby becomes an optional item that can appear at most once. Matches either one or zero instances of expr. For example, the following string matches a digit string of length one or greater. For example, the following string matches an empty string or a digit string: "*" For example, in the string “+”, “” is an expression and the + operator modifies its meaning.Īn operator associates with the character closest to it, except where brackets or parentheses are used, in which case the operator refers to the whole range or group that precedes it. In the following list, expr is an expression. This feature, as much as anything else, makes the regular-expression technology a powerful and versatile tool for searching and replacing text. Regular-expression pattern matching becomes more interesting when you modify a pattern to indicate possible repetitions. "^Barney"Įnd of line: The previous character matched (if any) must be the last character in the line of text. "\\Ba*"īeginning of line: The next character is matched only if it is the first character in the text to be examined or occurs just after a newline. For example, the following pattern matches a portion of a word beginning with “a.” It will match “at” embedded in “cat”, but it will not match “at” if it occurs as a stand-alone word. For example, the following string matches any word beginning with “c” and ending with “t”, such as “cat” or “containment”. Words are made up of alphanumeric characters and are delimited by whitespaces and punctuation.
![regex word boundary regex word boundary](https://i.stack.imgur.com/TurXp.png)
A word must begin or end at this position, or there is no match. Matches any character other than a digit, letter, or underscore. Matches any digit, letter, or underscore. Matches any character other than a whitespace character. Matches any character other than a digit. Matches a Unicode character specified as a hexadecimal code. Matches a character specified as a hexadecimal code. When using this in a C++ literal string meant to be part of a regular-expression pattern-matching string (as opposed to an actual embedded newline), remember that two backslashes must be used. For example, the following pattern string matches any single character other than “a”, “b”, or “c”: "" Matches any character not in the specified range. The following expression matches any one of the characters “+”, “*”, “/”, or “-”. The minus sign gains special meaning to indicate a run of characters as in the example just shown, but only if it appears between two other characters inside the range.
![regex word boundary regex word boundary](http://www.midnightdba.com/wp-content/uploads/2018/03/pjfdFQTgv4-1200x675.jpg)
![regex word boundary regex word boundary](https://data.templateroller.com/pdf_docs_html/485/4859/485970/python-php-perl-and-javascript-regex-cheat-sheet_print_big.png)
Most characters lose their special meaning inside the brackets. For example, the following pattern string matches any single letter in the range “a” to “m”. Matches any one character in the specified range. For example, the following pattern string matches almost any single character: Matches any one character other than a newline. For example, “” matches all lowercase letters. Characters are ordered according to their underlying numeric (ASCII) value. A range may optionally use a dash (minus sign) to indicate a run beginning with one character, up to and including another. In the following list, a range may be a list of characters (not separated by spaces or commas, which themselves are characters). This section also describes special conditions such as beginning-of-line or word boundary. The following special expressions match an individual character belonging to a group, such as letters or digits. Although it’s possible to use variations, the C++11 default is more versatile and expressive than the alternative grammars. This chapter adopts the default grammar used by the C++11 regular-expression library, which is a modified ECMAScript grammar. The next several sections summarize the syntax rules, beginning with the syntax for matching individual characters. The previous two sections provided an introduction to regular-expression patterns.