Regular Expressions Reference A regular expression is a pattern of text that consists of ordinary characters (for example, letters a through z) and special characters, known as metacharacters. The pattern describes one or more strings to match when searching a body of text. The regular expression serves as a template for matching a character pattern to the string being searched. Here are some some example Regular Expressions: Matches an HTML tag: `<(.*)>.*<\/\1>` Matches a complete line containing either "one", "two" or "three": `^.*(one|two|three).*$` Matches most email addresses: `.+@[^\.].+\.[a-zA-Z]{2,7}` Matches a date in the format yyyy-mm-dd, in between 1900-01-01 and 2099-12-31, with a choice of three separators (-/.): `(19|20)\d\d[-/.](0[1-9]|1[012])[-/.](0[1-9]|[12][0-9]|3[01])` The following table contains the complete list of metacharacters and their behavior in the context of regular expressions:
Character | Description
\ | Marks the next character as either a special character, a literal, a backreference, or an octal escape. For example, 'n' matches the character "n". '\n' matches a newline character. The sequence '\\' matches "\" and "\(" matches "(".
^ | Matches the position at the beginning of the input string.
$ | Matches the position at the end of the input string.
* | Matches the preceding subexpression zero or more times. For example, zo* matches "z" and "zoo". * is equivalent to {0,}.
+ | Matches the preceding subexpression one or more times. For example, 'zo+' matches "zo" and "zoo", but not "z". + is equivalent to {1,}.
? | Matches the preceding subexpression zero or one time. For example, "do(es)?" matches the "do" in "do" or "does". ? is equivalent to {0,1}
{n} | n is a nonnegative integer. Matches exactly n times. For example, 'o{2}' does not match the 'o' in "Bob," but matches the two o's in "food".
{n,} | n is a nonnegative integer. Matches at least n times. For example, 'o{2,}' does not match the "o" in "Bob" and matches all the o's in "foooood". 'o{1,}' is equivalent to 'o+'. 'o{0,}' is equivalent to 'o*'.
{n,m} | m and n are nonnegative integers, where n <= m. Matches at least n and at most m times. For example, "o{1,3}" matches the first three o's in "fooooood". 'o{0,1}' is equivalent to 'o?'. Note that you cannot put a space between the comma and the numbers.
? | When this character immediately follows any of the other quantifiers (*, +, ?, {n}, {n,}, {n,m}), the matching pattern is non-greedy. A non-greedy pattern matches as little of the searched string as possible, whereas the default greedy pattern matches as much of the searched string as possible. For example, in the string "oooo", 'o+?' matches a single "o", while 'o+' matches all 'o's.
. | Matches any single character except "\n". To match any character including the '\n', use a pattern such as '[.\n]'.
(pattern) | Matches pattern and captures the match for later use in a back reference.
(?:pattern) | Matches pattern but does not capture the match, that is, it is a non-capturing match that is not stored for possible later use. This is useful for combining parts of a pattern with the "or" character (|). For example, 'industr(?:y|ies) is a more economical expression than 'industry|industries'.
(?=pattern) | Positive lookahead matches the search string at any point where a string matching pattern begins. This is a non-capturing match, that is, the match is not captured for possible later use. For example 'Windows (?=95|98|NT|2000)' matches "Windows" in "Windows 2000" but not "Windows" in "Windows 3.1". Lookaheads do not consume characters, that is, after a match occurs, the search for the next match begins immediately following the last match, not after the characters that comprised the lookahead.
(?!pattern) | Negative lookahead matches the search string at any point where a string not matching pattern begins. This is a non-capturing match, that is, the match is not captured for possible later use. For example 'Windows (?!95|98|NT|2000)' matches "Windows" in "Windows 3.1" but does not match "Windows" in "Windows 2000". Lookaheads do not consume characters, that is, after a match occurs, the search for the next match begins immediately following the last match, not after the characters that comprised the lookahead.
x |y | Matches either x or y. For example, 'z|food' matches "z" or "food". '(z|f)ood' matches "zood" or "food".
[xyz] | A character set. Matches any one of the enclosed characters. For example, '[abc]' matches the 'a' in "plain".
[^xyz] | A negative character set. Matches any character not enclosed. For example, '[^abc]' matches the 'p' in "plain".
[a-z] | A range of characters. Matches any character in the specified range. For example, '[a-z]' matches any lowercase alphabetic character in the range 'a' through 'z'.
[^a-z] | A negative range characters. Matches any character not in the specified range. For example, '[^a-z]' matches any character not in the range 'a' through 'z'.
\b | Matches a word boundary, that is, the position between a word and a space. For example, 'er\b' matches the 'er' in "never" but not the 'er' in "verb".
\B | Matches a nonword boundary. 'er\B' matches the 'er' in "verb" but not the 'er' in "never".
\cx | Matches the control character indicated by x. For example, \cM matches a Control-M or carriage return character. The value of x must be in the range of A-Z or a-z. If not, c is assumed to be a literal 'c' character.
\d | Matches a digit character. Equivalent to [0-9].
\D | Matches a nondigit character. Equivalent to [^0-9].
\f | Matches a form-feed character. Equivalent to \x0c and \cL.
\n | Matches a newline character. Equivalent to \x0a and \cJ.
\r | Matches a carriage return character. Equivalent to \x0d and \cM.
\s | Matches any whitespace character including space, tab, form-feed, etc. Equivalent to [ \f\n\r\t\v].
\S | Matches any non-white space character. Equivalent to [^ \f\n\r\t\v].
\t | Matches a tab character. Equivalent to \x09 and \cI.
\v | Matches a vertical tab character. Equivalent to \x0b and \cK.
\w | Matches any word character including underscore. Equivalent to '[A-Za-z0-9_]'.
\W | Matches any nonword character. Equivalent to '[^A-Za-z0-9_]'.
\xn | Matches n, where n is a hexadecimal escape value. Hexadecimal escape values must be exactly two digits long. For example, '\x41' matches "A". '\x041' is equivalent to '\x04' & "1". Allows ASCII codes to be used in regular expressions.
\num | Matches num, where num is a positive integer. A reference back to captured matches. For example, '(.)\1' matches two consecutive identical characters.
\n | Identifies either an octal escape value or a backreference. If \n is preceded by at least n captured subexpressions, n is a backreference. Otherwise, n is an octal escape value if n is an octal digit (0-7).
\nm | Identifies either an octal escape value or a backreference. If \nm is preceded by at least nm captured subexpressions, nm is a backreference. If \nm is preceded by at least n captures, n is a backreference followed by literal m. If neither of the preceding conditions exists, \nm matches octal escape value nm when n and m are octal digits (0-7).
\nml | Matches octal escape value nml when n is an octal digit (0-3) and m and l are octal digits (0-7).