Python RegEx
A regular expression, or RegEx for short, is a series of characters that describes a search pattern.
With one, you can test whether a string contains that pattern.
RegEx Module
Regular expressions in Python are handled by re, a built-in package.
Start by importing it:
Example
main.py
import re
RegEx in Python
Once re is imported, regular expressions are ready to use:
Example
Check whether the string begins with "The" and finishes with "Spain":
Prompt
Import the
re module and search a string to see if it starts with "The" and ends with "Spain".
Use re.search() with the pattern "^The.*Spain$".
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)
RegEx Functions
To look for matches in a string, the re module gives you these functions:
findall- Gives back a list of every matchsearch- Gives back a Match object when a match exists anywhere in the stringsplit- Cuts the string at every match and gives back a list of the piecessub- Swaps one or many matches for a string of your choice
Metacharacters
A metacharacter is a character that carries a special meaning:
[]- Defines a set of characters\- Starts a special sequence, or escapes a special character.- Matches any character other than a newline^- Matches at the start$- Matches at the end*- Zero or more repeats+- One or more repeats?- Zero or one repeat{}- The exact number of repeats you specify|- One option or the other()- Captures and groups
Flags
Flags added to a pattern change how a regular expression behaves.
re.ASCII(re.A) - Matches ASCII characters onlyre.DEBUG- Shows debug informationre.DOTALL(re.S) - Lets the . character match everything, newlines includedre.IGNORECASE(re.I) - Ignores upper and lower case when matchingre.MULTILINE(re.M) - Matches at the start and end of every linere.NOFLAG- Says that this pattern has no flag setre.UNICODE(re.U) - Gives Unicode matchesre.VERBOSE(re.X) - Permits whitespace and comments inside a pattern
Special Sequences
Put a \ in front of certain characters and you get a special sequence, which has its own special meaning. These are the ones available:
\A- Finds a match only when the given characters open the string\b- Finds a match when the given characters begin or end a word\B- Finds a match when the given characters appear, but NOT at the beginning or end of a word\d- Finds a match for any digit (0-9) in the string\D- Finds a match for anything in the string that is NOT a digit\s- Finds a match for any white space character in the string\S- Finds a match for anything in the string that is NOT white space\w- Finds a match for any word character in the string\W- Finds a match for anything in the string that is NOT a word character\Z- Finds a match only when the given characters close the string
Sets
A set is a group of characters between square brackets [], and it too has a special meaning:
[arn]- Matches one of the listed characters: a, r or n[a-n]- Matches any lowercase letter that falls between a and n in the alphabet[^arn]- Matches any character other than a, r and n[0123]- Matches one of the listed digits: 0, 1, 2 or 3[0-9]- Matches any single digit from 0 through 9[0-5][0-9]- Matches any two-digit number from 00 through 59[a-zA-Z]- Matches any letter from a to z, whether lower case OR upper case[+]- Inside a set, the characters +, *, ., |, (), $ and {} lose their special meaning, so [+] just looks for a literal + in the string
The findall() Function
The findall() function collects all the matches into a list, which keeps them in the order they were found.
When nothing matches, you get an empty list:
Example
Show every match as a list:
Prompt
Use
re.findall() to find all occurrences of "ai" in "The rain in Spain".
Print the list of matches.
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)
Example
With no match to collect, the list comes back empty:
Prompt
Use
re.findall() to search for "Portugal" in "The rain in Spain".
Print the result (empty list when no match).
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.findall("Portugal", txt)
print(x)
The search() Function
The search() function looks through the string and, when it finds a match, gives you a Match object.
Several matches in the string make no difference, because you only get the first one:
When nothing matches, you get the value None:
Example
Find where the first white-space character sits in the string:
Prompt
Use
re.search() to find the first white-space character in "The rain in Spain".
Print its position using .start().
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.search("\s", txt)
print("The first white-space character is located in position:", x.start())
Example
Run a search that finds nothing:
Prompt
Use
re.search() to search for "Portugal" in "The rain in Spain".
Print the result (None when no match).
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.search("Portugal", txt)
print(x)
The split() Function
The split() function breaks the string at each match and returns the pieces in a list:
The maxsplit parameter limits how many splits are made:
Example
Break the string at every white-space character:
Prompt
Use
re.split() to split "The rain in Spain" at each white-space character.
Print the resulting list.
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.split("\s", txt)
print(x)
Example
Break the string at the first white-space character only:
Prompt
Use
re.split() to split "The rain in Spain" at the first occurrence only (maxsplit=1).
Print the result.
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.split("\s", txt, 1)
print(x)
The sub() Function
The sub() function puts text of your choosing in place of each match:
The count parameter limits how many replacements are made:
Example
Put the number 9 wherever there is a white-space character:
Prompt
Use
re.sub() to replace every white-space with "9" in "The rain in Spain".
Print the result.
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x)
Example
Change only the first 2 white-space characters:
Prompt
Use
re.sub() to replace the first 2 white-space characters with "9" in "The rain in Spain".
Print the result.
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.sub("\s", "9", txt, 2)
print(x)
Match Object
A Match Object holds details about a search and what it found.
Note: When nothing matches, you get the value None and no Match Object at all.
To get at those details, use the object's properties and methods:
.span()gives a tuple holding where the match starts and where it ends.stringgives back the string you passed to the function.group()gives back the matching text
Example
Run a search that produces a Match Object:
Prompt
Use
re.search() to search for "ai" in "The rain in Spain".
Print the Match object returned.
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.search("ai", txt)
print(x)
Example
Show the start and end positions of the first match:
Prompt
Search for a word starting with "S" using
r"\bS\w+" in "The rain in Spain".
Print the span of the match using .span().
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span())
Example
Show the string that was given to the function:
Prompt
Search for a word starting with "S" using
r"\bS\w+" in "The rain in Spain".
Print the original string using .string.
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.string)
Example
Show just the matching text:
Prompt
Search for a word starting with "S" using
r"\bS\w+" in "The rain in Spain".
Print the matched text using .group().
Output: complete Python program
main.py
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.group())