Metacharacters
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Published Jun 10, 2022Updated Sep 5, 2023
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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$' matches “Codecademy.” |
Quantifiers
Some metacharacters are concerned with the frequency of certain character patterns as shown in the table below:
| Metacharacter | Description | Example |
|---|---|---|
? |
Matches zero or one of the preceding character. | r'neighbo?ur' matches “neighbor” and “neighbour.” |
* |
Matches zero or more of the preceding character. | r're*d' matches “red” and “reed.” |
+ |
Matches one or more of the preceding character. | r'tw+o' matches “two” but not “to.” |
| |
Matches either the pattern before or after the |. |
r'true|false' matches “true“ or “false.” |
{x} |
Matches if the preceding character occurred x times in a row. |
r're{2}d' matches “reed” (2 “e”s) but not “red” (only 1 “e”). |
Capture Groups
Capture groups can be used to check and quantify different patterns in the string. This can be done with parentheses (...), as shown in the example below:
import repattern_one = 'red'pattern_two = 'rad'pattern_three = 'rid'capture_group = r'r(e|a)d'print(re.match(capture_group, pattern_one))print(re.match(capture_group, pattern_two))print(re.match(capture_group, pattern_three))
The capture group uses the pipe | quantifier to match two out of the three patterns since they both have “e” or an “a”. The following output will be printed:
<_sre.SRE_Match object; span=(0, 3), match='red'><_sre.SRE_Match object; span=(0, 3), match='rad'>None
Special Sequences
The backslash \ metacharacter has two primary uses:
- It matches a character literal that would otherwise have meaning (e.g.
^or$). - 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.” |
Codebyte Example
The following snippet can be used for practicing with regex metacharacters:
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