[950] Python RegEx (re library)

ref: Python RegEx

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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.

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RegEx Module

Python has a built-in package called re, which can be used to work with Regular Expressions.

RegEx in Python

After importing the re module, we can start using regular expressions:

RegEx Functions

The re module offers a set of functions that allows us to search a string for a match:

Function	Description
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

Here are some exmaples:

a = "Today is sunny day and I am working now."
 
r1 = re.findall("\w ", a) 
r2 = re.search("\w ", a) 
r3 = re.split("\w ", a) 
r4 = re.sub("\w ", "==", a) 
 
print(r1) 
print(r2)
print(r3)  
print(r4) 
 
# ['y ', 's ', 'y ', 'y ', 'd ', 'I ', 'm ', 'g ']
# <re.Match object; span=(4, 6), match='y '>
# ['Toda', 'i', 'sunn', 'da', 'an', '', 'a', 'workin', 'now.']
# Toda==i==sunn==da==an====a==workin==now.

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Metacharacters

Metacharacters are characters with a special meaning:

Character	Description	Example	Try it
[]	A set of characters	"[a-m]"	Try it »
\	Signals a special sequence (can also be used to escape special characters)	"\d"	Try it »
.	Any character (except newline character)	"he..o"	Try it »
^	Starts with	"^hello"	Try it »
$	Ends with	"planet$"	Try it »
*	Zero or more occurrences	"he.*o"	Try it »
+	One or more occurrences	"he.+o"	Try it »
?	Zero or one occurrences	"he.?o"	Try it »
{}	Exactly the specified number of occurrences	"he.{2}o"	Try it »
|	Either or	"falls|stays"	Try it »
()	Capture and group

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Character	Description	Example	Try it
\A	Returns a match if the specified characters are at the beginning of the string	"\AThe"	Try it »
\b	Returns a match where the specified characters are at the beginning or at the end of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\bain" r"ain\b"	Try it » Try it »
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\Bain" r"ain\B"	Try it » Try it »
\d	Returns a match where the string contains digits (numbers from 0-9)	"\d"	Try it »
\D	Returns a match where the string DOES NOT contain digits	"\D"	Try it »
\s	Returns a match where the string contains a white space character	"\s"	Try it »
\S	Returns a match where the string DOES NOT contain a white space character	"\S"	Try it »
\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"	Try it »
\W	Returns a match where the string DOES NOT contain any word characters	"\W"	Try it »
\Z	Returns a match if the specified characters are at the end of the string	"Spain\Z"	Try it »

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Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Set	Description	Try it
[arn]	Returns a match where one of the specified characters (a, r, or n) is present	Try it »
[a-n]	Returns a match for any lower case character, alphabetically between a and n	Try it »
[^arn]	Returns a match for any character EXCEPT a, r, and n	Try it »
[0123]	Returns a match where any of the specified digits (0, 1, 2, or 3) are present	Try it »
[0-9]	Returns a match for any digit between 0 and 9	Try it »
[0-5][0-9]	Returns a match for any two-digit numbers from 00 and 59	Try it »
[a-zA-Z]	Returns a match for any character alphabetically between a and z, lower case OR upper case	Try it »
[+]	In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string	Try it »

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