Learn and Test Regular Expressions Freely

The RegEx Lab in Mergemill Pro is a regex checker that lets you test and learn powerful regular expressions

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abc

Matches all of a, b, and c in order. For example, regex "123" matches "01123345".

a|b|c

Matches one of a, b, or c. For example, regex "1|2|3" matches "01123345".

[a-z0-9]

Matches any single character of the set enclosed in square brackets. Examples: [aeiou] matches any one of the vowels. [a-zA-Z0-9] matches any alphanumeric character. [a-e] matches any character in the range a-e, inclusive. To match a "-", place it at the beginning or end of the set. For example, [a-c-] finds a character in the range a-c or the "-" sign. Other useful patterns are: "[[]" finds a "[". "[]]" finds a "]".

[^a-z0-9]

Matches any single character NOT in the set. For example, [^aeiou] matches any character EXCEPT a vowel. To find the caret character, place it anywhere except the first position after the opening bracket. For example, [a-e^] finds a character in the range a-e or the caret character.

\d

Matches a digit. Same as [0-9].

\D

Matches a non-digit. Same as [^0-9].

\w

Matches an alphanumeric (word) character. Same as [a-zA-Z0-9_].

\W

Matches a non-word character. Same as [^a-zA-Z0-9_].

\s

Matches a whitespace character (space, tab, return, line feed, form feed).

\S

Matches a non-whitespace character. Please note that [\D\S] is NOT the same as [^\d\s]. In fact, [\D\S] matches ANYTHING.

\n

Matches a newline (or line feed).

\r

Matches a return.

\t

Matches a tab.

\f

Matches a formfeed.

\0

Matches a null character.

\000

Also matches a null character. This is a specific case of \nnn.

\nnn

Matches an ASCII character of the octal value nnn. "\15" is the same as "\r".

\xnn

Matches an ASCII character of the hexadecimal value nn. So another way of searching for the return character is to use \xD.

\cX

Matches an ASCII control character. The letter after the backslash is always a lowercase c. The second letter is an uppercase letter A through Z, to indicate Control+A through Control+Z. These are equivalent to \x01 through \x1A.

\metachar

Matches the metacharacter, such as \., \\, and \|.

. (dot)

Matches any character except a line break. If you use the dot alone, you will select the first character in the target string and, if you repeat the search, you will find each successive character, till you encounter a line break. For example, "5.." matches "0123456789". The dot means [^\n] in Unix, [^\r\n] in Windows, and [^\r] in Mac OS. Don't use the dot if you can; your regex is more efficient if you specify more clearly the strings you want to match. Optimizing a regex is important if it is to be used repeatedly and on large chunks of data.

^

Matches the beginning of a line or string. For example, "^Name" finds lines that begin with "Name".

$

Matches the end of a line or string. For example, ".$" finds the last character in a line.

\b

Matches a word boundary. For example, "\bword\b" does a whole-word search.

\B

Matches a non-word boundary. It matches where \b does not.

x?

Repeats x zero or one time. That is, x is optional in the strings to be matched. For example, "12?3" matches both "0123456789" and "013456789".

x*

Repeats x zero or more times in the strings to be matched. For example, "12*" matches "01222223456789".

x+

Repeats x one or more times in the strings to be matched. For example, [0-9]+ finds a string of one or more consecutive numbers, such as "32" in "Win32".

x{m,n}

Repeats x m to n times in the strings to be matched.

x{n}

Repeats x exactly n times in the strings to be matched.

x{n,}

Repeats x at least n times in the strings to be matched.

$'

Replaced with the entire target string following the matched text.

$&

Same as \0 or $0, it contains the entire matched string. For example, if "\d\d\d\d\sB\.C\." finds "1541 B.C.", then the replacement pattern "the year $&" results in "the year 1541 B.C.".

$0-$50

Same as \0 to \50. They evaluate to nothing if the subexpression corresponding to the number doesn't exist, otherwise they contain the last-matched subpatterns, defined by the parentheses in the search pattern.

\xnn

Replaced with the character represented by nn in Hex.

\nnn

Replaced with the character represented by nn in Octal.

\cX

Replaced with the character that is the control version of X.

(?#text)

Use this to insert a comment.

(?:regex)

This is for grouping without creating backreferences, and is therefore empty when called.

(?=regex)

This is a zero-width positive look-ahead assertion. For example, \w+(?=\t) matches a word followed by a tab, without including the tab in $&.

(?!regex)

This is a zero-width negative look-ahead assertion. For example foo(?!bar) matches any occurrence of "foo" that isn't followed by "bar".

(?<=regex)

This is a zero-width positive look-behind assertion. For example, (?<=\t)\w+ matches a word that follows a tab, without including the tab in $&. It works only for fixed-width look-behind regex.

(?<!regex)

This is a zero-width negative look-behind assertion. It works only for a fixed-width look-behind regex. For example "\b\w+(?<!s)\b" finds a word that does not end with an "s". Without using lookbehind, the regex becomes \b\w*[^s\W]\b