# Regular Expressions Cheat Sheet

A regular expression (regex or regexp) is a sequence of characters that specifies a search pattern. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation.

The below tables assists you on how to use different regular expressions:

| **Character** | **Explanation**                        |
| ------------- | -------------------------------------- |
| .             | Anything. Any character except newline |
| a             | The character a                        |
| ab            | The string ab                          |
| a\|b          | a or b                                 |
| a\*           | 0 or more a's                          |
| \\            | Escapes a special character            |

## Regular Expression Quantifiers

| **Character** | **Explanation** |
| ------------- | --------------- |
| \*            | 0 or more       |
| +             | 1 or more       |
| ?             | 0 or 1          |
| {2}           | Exactly 2       |
| {2, 5}        | Between 2 and 5 |
| {2,}          | 2 or more       |

## Regular Expression Groups

| **Character** | **Explanation**                        |
| ------------- | -------------------------------------- |
| (...)         | Capturing group                        |
| (?P\<Y>...)   | Capturing group named Y                |
| (?:...)       | Non-capturing group                    |
| (?>...)       | Atomic group                           |
| (?\|...)      | Duplicate group numbers                |
| \Y            | Match the Y'th captured group          |
| (?P=Y)        | Match the named group Y                |
| (?R)          | Recurse into entire pattern            |
| (?Y)          | Recurse into numbered group Y          |
| (?\&Y)        | Recurse into named group Y             |
| \g{Y}         | Match the named or numbered group Y    |
| \g\<Y>        | Recurse into named or numbered group Y |
| (?#...)       | Comment                                |

## Regular Expression Character Classes

| **Character** | **Explanation**                  |
| ------------- | -------------------------------- |
| \[ab-d]       | One character of: a, b, c, d     |
| \[^ab-d]      | One character except: a, b, c, d |
| \[\b]         | Backspace character              |
| \d            | One digit                        |
| \D            | One non-digit                    |
| \s            | One whitespace                   |
| \S            | One non-whitespace               |
| \w            | One word character               |
| \W            | One non-word character           |

## Regular Expression Assertions

| **Character** | **Explanation**                 |
| ------------- | ------------------------------- |
| ^             | Start of string                 |
| \A            | Start of string, ignores m flag |
| $             | End of string                   |
| \Z            | End of string, ignores m flag   |
| \b            | Word boundary                   |
| \B            | Non-word boundary               |
| \G            | Start of match                  |
| (?=...)       | Positive lookahead              |
| (?!...)       | Negative lookahead              |
| (?<=...)      | Positive lookbehind             |
| (?\<!...)     | Negative lookbehind             |
| (?()\|)       | Conditional                     |

## Regular Expression Flags

| **Character** | **Explanation**                     |
| ------------- | ----------------------------------- |
| i             | Ignore case                         |
| m             | ^ and $ match start and end of line |
| s             | . matches newline as well           |
| x             | Allow spaces and comments           |
| J             | Duplicate group names allowed       |
| U             | Ungreedy quantifiers                |
| (?iLmsux)     | Set flags within regex              |

## Regular Expression Special Characters

| **Character** | **Explanation**           |
| ------------- | ------------------------- |
|               | Newline                   |
|               | Carriage return           |
|               | Tab                       |
| \0            | Null character            |
| \YYY          | Octal character YYY       |
| \xYY          | Hexadecimal character YY  |
| \x{YY}        | Hexadecimeal character YY |
| \cY           | Control character Y       |

## Regular Expression Posix Classes

| **Character** | **Explanation**                  |
| ------------- | -------------------------------- |
| \[:alnum:]    | Letters and digits               |
| \[:alpha:]    | Letters                          |
| \[:ascii:]    | Ascii codes 0 - 127              |
| \[:blank:]    | Space or tab only                |
| \[:cntrl:]    | Control characters               |
| \[:digit:]    | Decimal digits                   |
| \[:graph:]    | Visible characters, except space |
| \[:lower:]    | Lowercase letters                |
| \[:print:]    | Visible characters               |
| \[:punct:]    | Visible punctuation characters   |
| \[:space:]    | Whitespace                       |
| \[:upper:]    | Uppercase letters                |
| \[:word:]     | Word characters                  |
| \[:xdigit:]   | Hexadecimal digits               |


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