Python RegEx
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.
RegEx Module
Python has a built-in package called re , which can be used to work with Regular Expressions.
RegEx in Python
When you have imported the re module, you can start using regular expressions:
Example
Search the string to see if it starts with «The» and ends with «Spain»:
txt = «The rain in Spain»
x = re.search(«^The.*Spain$», txt)
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 |
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.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 » |
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 » |
1 | Returns a match for any digit between 0 and 9 | Try it » |
19 | 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 » |
The findall() Function
The findall() function returns a list containing all matches.
Example
Print a list of all matches:
txt = «The rain in Spain»
x = re.findall(«ai», txt)
print(x)
The list contains the matches in the order they are found.
If no matches are found, an empty list is returned:
Example
Return an empty list if no match was found:
txt = «The rain in Spain»
x = re.findall(«Portugal», txt)
print(x)
The search() Function
The search() function searches the string for a match, and returns a Match object if there is a match.
If there is more than one match, only the first occurrence of the match will be returned:
Example
Search for the first white-space character in the string:
txt = «The rain in Spain»
x = re.search(«\s», txt)
print(«The first white-space character is located in position:», x.start())
If no matches are found, the value None is returned:
Example
Make a search that returns no match:
txt = «The rain in Spain»
x = re.search(«Portugal», txt)
print(x)
The split() Function
The split() function returns a list where the string has been split at each match:
Example
Split at each white-space character:
txt = «The rain in Spain»
x = re.split(«\s», txt)
print(x)
You can control the number of occurrences by specifying the maxsplit parameter:
Example
Split the string only at the first occurrence:
txt = «The rain in Spain»
x = re.split(«\s», txt, 1)
print(x)
The sub() Function
The sub() function replaces the matches with the text of your choice:
Example
Replace every white-space character with the number 9:
txt = «The rain in Spain»
x = re.sub(«\s», «9», txt)
print(x)
You can control the number of replacements by specifying the count parameter:
Example
Replace the first 2 occurrences:
txt = «The rain in Spain»
x = re.sub(«\s», «9», txt, 2)
print(x)
Match Object
A Match Object is an object containing information about the search and the result.
Note: If there is no match, the value None will be returned, instead of the Match Object.
Example
Do a search that will return a Match Object:
txt = «The rain in Spain»
x = re.search(«ai», txt)
print(x) #this will print an object
The Match object has properties and methods used to retrieve information about the search, and the result:
.span() returns a tuple containing the start-, and end positions of the match.
.string returns the string passed into the function
.group() returns the part of the string where there was a match
Example
Print the position (start- and end-position) of the first match occurrence.
The regular expression looks for any words that starts with an upper case «S»:
txt = «The rain in Spain»
x = re.search(r»\bS\w+», txt)
print(x.span())
Example
Print the string passed into the function:
txt = «The rain in Spain»
x = re.search(r»\bS\w+», txt)
print(x.string)
Example
Print the part of the string where there was a match.
The regular expression looks for any words that starts with an upper case «S»:
txt = «The rain in Spain»
x = re.search(r»\bS\w+», txt)
print(x.group())
Note: If there is no match, the value None will be returned, instead of the Match Object.