They can also be used in the replacement string or retrieved in the match_results object filled by some regex operations.Īssertions are conditions that do not consume characters in the target sequence: they do not describe a character, but a condition that must be fulfilled before or after a character.Įither it is the beginning of the target sequence, or follows a line terminator.Įither it is the end of the target sequence, or precedes a line terminator. These submatches can be used in the regular expression itself to specify that the entire subpattern should appear again somewhere else (see \ int in the special characters list). Each submatch is numbered after the order of appearance of their opening parenthesis (the first submatch is number 1, the second is number 2, and so on.).
When a group creates a backreference, the characters that represent the subpattern in the target sequence are stored as a submatch. Groups allow to apply quantifiers to a sequence of characters (instead of a single character). While matching "(a+?).*" against "aardvark" also succeeds, but yields a as the first submatch.
Matching "(a+).*" against "aardvark" succeeds and yields aa as the first submatch. This behavior can be overridden to ungreedy (i.e., take as few characters that meet the condition as possible) by adding a question mark ( ?) after the quantifier. The preceding atom is matched at least min times, but not more than max.īy default, all these quantifiers are greedy (i.e., they take as many characters that meet the condition as possible). The preceding atom is matched int or more times.
The preceding atom is matched exactly int times. The preceding atom is optional (matched either 0 times or once). The preceding atom is matched 1 or more times. The preceding atom is matched 0 or more times. They can modify the amount of times that character is repeated in the match: Quantifiers follow a character or a special pattern character. Std::regex e2 ( "\\\\") // regular expression: \\ -> matches a single backslash (\) character Std::regex e1 ( "\\d") // regular expression: \d -> matches a digit character Notice that, in C++, character and string literals also escape characters using the backslash character ( \), and this affects the syntax for constructing regular expressions from such types. The target character is not part of the class (see character classes below) The target character is part of the class (see character classes below) The character character as it is, without interpreting its special meaning within a regex expression.Īny character can be escaped except those which form any of the special character sequences above. See groups below for more info.Ī decimal digit character (same as ]).Īny character that is not a decimal digit character (same as ]).Ī whitespace character (same as ]).Īny character that is not a whitespace character (same as ]).Īn alphanumeric or underscore character (same as ]).Īny character that is not an alphanumeric or underscore character (same as ]). The result of the submatch whose opening parenthesis is the int-th ( int shall begin by a digit other than 0).
#REGEX SPECIAL CHARACTERS CODE#
Each of these special pattern characters is matched in the target sequence against a single character (unless a quantifier specifies otherwise).Īny character except line terminators (LF, CR, LS, PS).Ī horizontal tab character (same as \u0009).Ī newline (line feed) character (same as \u000A).Ī vertical tab character (same as \u000B).Ī carriage return character (same as \u000D).Ī control code character whose code unit value is the same as the remainder of dividing the code unit value of letter by 32.įor example: \ca is the same as \u0001, \cb the same as \u0002, and so on.Ī character whose code unit value has an hex value equivalent to the two hex digits hh.įor example: \x4c is the same as L, or \x23 the same as #.Ī character whose code unit value has an hex value equivalent to the four hex digits hhhh. Special pattern characters are characters (or sequences of characters) that have a special meaning when they appear in a regular expression pattern, either to represent a character that is difficult to express in a string, or to represent a category of characters. But the regex syntax allows for special characters and expressions in the pattern: Regular expression operations look sequentially for matches between the characters of the pattern and the characters in the target sequence: In principle, each character in the pattern is matched against the corresponding character in the target sequence, one by one. The following syntax is used to construct regex objects (or assign) that have selected ECMAScript as its grammar.Ī regular expression pattern is formed by a sequence of characters. ECMAScript regular expressions pattern syntax
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