A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern. RegEx can be used to check if a string contains the specified search pattern.
Python has a built-in package called re, which can be used to work with Regular Expressions. Import the re module: import re
When you have imported the re module, you can start using regular expressions:
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The re module offers a set of functions that allows us to search a string for a match :
Function | Description |
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findall | Returns a list containing all matches |
search | Returns a Match object if there is a match anywhere in the string |
split | Returns a list where the string has been split at each match |
sub | Replaces one or many matches with a string |
Metacharacters are characters with a special meaning :
Character | Description | Example |
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[ ] | A set of characters | "[a-m]" |
\ | Signals a special sequence (can also be used to escape special characters) | "\d" |
. | Any character (except newline character) | "he..o" |
^ | Starts with | "^hello" |
$ | Ends with | "world$" |
* | Zero or more occurrences | "aix*" |
+ | One or more occurrences | "aix+" |
{ } | Excactly the specified number of occurrences | "al{2}" |
| | Either or | "al{2}" |
( ) | Capture and group | "falls|stays" |
A special sequence is a \ followed by one of the characters in the list below, and has a special meaning :
Character | Description | Example |
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\A | Returns a match if the specified characters are at the beginning of the string | "\AThe" |
\b | Returns a match where the specified characters are at the beginning or at the end of a word | r"\bmazon" r"zon\b" |
\B | Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word | r"\Bmazon" r"zon\B" |
\d | Returns a match where the string contains digits (numbers from 0-9) | "\d" |
\D | Returns a match where the string DOES NOT contain digits | "\D" |
\s | Returns a match where the string contains a white space character | "\s" |
\S | Returns a match where the string DOES NOT contain a white space character | "\S" |
\w | Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character) | "\w" |
\W | Returns a match where the string DOES NOT contain any word characters | "\W" |
\Z | Returns a match if the specified characters are at the end of the string | "Amazon\Z" |
A set is a set of characters inside a pair of square brackets [ ] with a special meaning:
Set | Description |
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[arn] | Returns a match where one of the specified characters (a, r, or n) are present |
[a-n] | Returns a match for any lower case character, alphabetically between a and n |
[^arn] | Returns a match for any character EXCEPT a, r, and n |
[0123] | Returns a match where any of the specified digits (0, 1, 2, or 3) are present |
[0-9] | Returns a match for any digit between 0 and 9 |
[0-5][0-9] | Returns a match for any two-digit numbers from 00 and 59 |
[a-zA-Z] | Returns a match for any character alphabetically between a and z, lower case OR upper case |
[0123] | Returns a match where any of the specified digits (0, 1, 2, or 3) are present |
[+] | In sets, +, *, ., |, ( ), $,{ } has no special meaning, so [+] means: return a match for any + character in the string |
The findall() function returns a list containing all matches.
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The search() function searches the string for a match, and returns a Match object if there is a match.
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Note : If no matches are found, the value None is returned
The split() function returns a list where the string has been split at each match.
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The sub() function replaces the matches with the text of your choice.
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