Regular Expressions

Published Jun 3, 2022Updated Oct 26, 2022

Regular expressions, often shortened to regex or regexp, is a language used for pattern-matching text content. It is implemented in several different programming languages, either directly or through libraries. Languages that implement regular expressions include Python, Java, JavaScript, C and C++. Regular expressions are generally standardized, though some implementations may provide some additions and variations to the basic syntax.

Implementation in each programming language is generally through functions and methods that accept some text and a regular expression pattern, and return a result based on what the regular expression pattern matched in the text.

Regular Expression Patterns

A regular expression pattern is a string made up of normal characters mixed with metacharacters that have special meanings defining how characters should be matched. For instance, the pattern “h” by itself would match the single letter “h”, whereas if we add the metacharacter “.”, which matches any one character, then “h.” would match sequences such as “ha”, “ho”, “h5” and “h!”.

Regular expressions use the following metacharacters and sequences for pattern matching:

Metacharacter	Description	Example
`^`	Matches the start of text, or the start of a line, depending on the settings of the regex engine.	`^a` only matches the first “a” in “a match is made”
`$`	Matches the end of text, or the end of a line, depending on the settings of the regex engine.	`!$` only matches the second “!” in “Hello! World!”
`.`	Matches any one character.	`.` matches “c”, “a” and “t” in “cat”
`*`	Matches the preceding element zero or more times.	`co*t` matches “ct”, “cot” and “coot”
`+`	Matches the preceding element one or more times.	`co+t` matches “cot”, “coot” but not “ct”
`?`	Matches the preceding element zero or one time.	`co?t` matches “ct”, “cot” but not “coot”
`{n}`	Matches the preceding element exactly `n` times.	`co{2}t` matches “coot” but not “cot” or “cooot”
`{,max}`	Matches the preceding element zero to `max` times.	`co{,2]t` matches “ct”, “cot”, “coot” but not “cooot”
`{min,}`	Matches the preceding element `min` or more times.	`co{2,}t` matches “coot” and “cooot” but not “cot”
`{m,n}`	Matches the preceding element at least `m` and not more than `n` times.	`co{2,3}t` matches “coot” and “cooot” but not “cot”
`[ ]`	Matches the characters inside the brackets. Can also specify ranges of characters such as `[a-z]`.	`[bcm]at` matches “cat”, “bat” and “mat”
`[^ ]`	Matches characters not inside the brackets. Can also specify ranges of characters such as `[^a-z]`.	`b[^a]t` matches “bit”, “bot”, and “but” but not “bat”
`\|`	Matches the pattern on the right of the `\|` or the pattern on the left.	`yes\|no` matches both “yes” and “no”
`( )`	Groups a pattern together and marks it for future reference	`(ca\|bi)t` matches “cat” and “bit”
`\w`	Matches alphanumeric characters and “_“.	`\w+` matches “Hello” and “World” in “Hello World!”
`\W`	Matches non-word characters.	`\W` matches the space and “!” in “Hello World!”
`\b`	Matches word boundaries.	`o\b` matches the first “o” in “Hello World!”
`\B`	Matches non-word boundaries.	`o\B` matches the second “o” in “Hello World!”
`\d`	Matches a digit character.	`\d+` matches “123” in “abc123”
`\D`	Matches a non-digit character.	`\D+` matches “abc” in “abc123”
`\s`	Matches a whitespace character.	`\s` matches the space in “Hello World!”
`\S`	Matches a non-whitespace character.	`\S+` matches “Hello” and “World!” in “Hello World!”