Metacharacters

In regex, certain metacharacters are used to match and qualify regular character patterns or other expressions.

Common Metacharacters

Metacharacter	Description	Example
`.`	Matches any character.	`r'.'` matches “[email protected]“.
`[]`	Matches character class inside the brackets not excluded by `^`.	`r'Char[mander\|meleon\|izard]'` matches “Charmander“, “Charmeleon“, and “Charizard.”
`^`	Matches characters at the beginning of a string.	`r'^C'` matches “Codecademy.”
`$`	Matches characters at the end of a string.	`r'y$'` matches “Codecademy.”

Quantifiers

Some metacharacters are concerned with the frequency of certain character patterns as shown in the table below:

Metacharacter	Description	Example
`?`	Matches zero or one of the preceding character.	`r'neighbo?ur'` matches “neighbor” and “neighbour.”
`*`	Matches zero or more of the preceding character.	`r're*d'` matches “red” and “reed.”
`+`	Matches one of more of the preceding character.	`r'tw+o'` matches “two” but not “to.”
`\|`	Matches either the pattern before or after the `\|`.	`r'true\|false'` matches “true“ or “false.”
`{x}`	Matches if the preceding character occurred `x` times in a row.	`r're{2}d'` matches “reed” (2 “e”s) but not “red” (only 1 “e”).

Capture Groups

Capture groups can be used to check and quantify different patterns in the string. This can be done with parentheses (...), as shown in the example below:

import re

pattern_one = 'red'
pattern_two = 'rad'
pattern_three = 'rid'

capture_group = r'r(e|a)d'

print(re.match(capture_group, pattern_one))
print(re.match(capture_group, pattern_two))
print(re.match(capture_group, pattern_three))

The capture group uses the pipe | quantifier to match two out of the three patterns since they both have “e” or an “a”. The following output will be printed:

<_sre.SRE_Match object; span=(0, 3), match='red'>
<_sre.SRE_Match object; span=(0, 3), match='rad'>
None

Special Sequences

The backslash \ metacharacter has two primary uses:

It matches a character literal that would otherwise have meaning (e.g. ^ or $).
It finds a match within particular character classes or sequences, such as digits.

The table below describes these special metacharacters when used with the \ backslash:

Metacharacter	Description	Example
`\A`	Only matches the beginning of a string.	`r'\AC'` matches “Codecademy” but not “codecademy.”
`\b`	Matches the boundary at the beginning or end of a string.	`r'\bCode\b'` matches “Code Ninja” but not “CodeNinja.”
`\B`	Matches the boundary within a string.	`r'Code\B'` matches “CodeNinja” but not Code Ninja.”
`\d`	Matches any digit character (0-9) in a string.	`r'\d square'` matches “4 square” but not “four square.”
`\D`	Matches any non-digit character in a string.	`'\D square'` matches “four square” but not “4 square”.
`\s`	Matches any whitespace character including tabs and line breaks.	`r'Code\sNinja'` matches “Code Ninja” but not “CodeNinja.”
`\S`	Matches any non-whitespace character.	`r'Code Ninja\S'` matches “Code Ninjas“ but not “Code Ninja.”
`\w`	Matches most word characters including numbers and the underscore.	`r'\w'` matches everything in “code_ninja.txt” except for the period `.`.
`\W`	Matches any non-word character.	`r'\W'` only matches the period `.` in “code_ninja.txt.”
`\Z`	Only matches the end of a string.	`r'emy\Z'` matches “Codecademy“ but not “CODECADEMY.”