Clojure - Regular Expressions

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A regular expression is a pattern that is used to find substrings in text. Regular expressions are used in a variety of programming languages and used a lot in LISP type programming languages.

Following is an example of a regular expression.

//d+

The above regular expression is used to find one more occurrence of a digit in a string. The // characters are used to ensure that characters ‘d’ and ‘+’ are used to represent a regular expression.

In general, regular expressions works with the following set of rules.

  • There are two special positional characters that are used to denote the beginning and end of a line: caret (∧) and dollar sign ($):

  • Regular expressions can also include quantifiers. The plus sign (+) represents one or more times, applied to the preceding element of the expression. The asterisk (*) is used to represent zero or more occurrences. The question mark (?) denotes zero or once.

  • The metacharacter { and } is used to match a specific number of instances of the preceding character.

  • In a regular expression, the period symbol (.) can represent any character. This is described as the wildcard character.

  • A regular expression may include character classes. A set of characters can be given as a simple sequence of characters enclosed in the metacharacters [and] as in [aeiou]. For letter or number ranges, you can use a dash separator as in [a–z] or [a–mA–M]. The complement of a character class is denoted by a leading caret ithin the square brackets as in [∧a–z] and represents all characters other than those specified.

The following methods are available for regular expressions.

Sr.No. Methods & Description
1 re-pattern

Returns an instance of java.util.regex.Pattern. This is then used in further methods for pattern matching.
2 refind

Returns the next regex match, if any, of string to pattern, using java.util.regex.Matcher.find()
3 replace

The replace function is used to replace a substring in a string with a new string value. The search for the substring is done with the use of a pattern.
4 replace-first

The replace function is used to replace a substring in a string with a new string value, but only for the first occurrence of the substring. The search for the substring is done with the use of a pattern.
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