Regex python module

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

It is useful for mainly two tasks:

1. Verifying that strings match a pattern (for instance, that a string has the format of an email address).
2. Performing substitutions in a string (such as changing all American spelling to British ones).

Some basics

The below example checks whether the pattern "spam" matches the string and prints "Match if it does.

import re

# Define a regular expression
pattern = r"spam"

# Run 're.match' function to determine whether it matches at the beginning of a string.
if re.match(pattern, "spamspamspam"):
    print("Match")
else:
    print("No Match")



>>>
Match
>>>

Other functions we can use to match patterns are :

re.search: finds a match of a pattern anywhere in the string.

re.findall: returns a list of all substrings that match a pattern.

if re.match(pattern, "eggspamsausagespam"):
    print("Match")
else:
    print("No match")

if re.search(pattern, "eggspamsausagespam"):
    print("Match")
else:
    print("No Match")


print(re.findall(pattern, "eggspamsausagespam"))


>>>
No match
Match
['spam', 'spam']
>>>

The regex search returns an object with several methods that give details about it.

group: returns the string matched.

start/end: returns the start and ending postions of the first match respectively.

span: returns the starts and end positions of the first match as a tuple.

import re

pattern = r"pam"

match = re.search(pattern, "eggspamsausage")

if match:
    print(match.group())
    print(match.start())
    print(match.end())
    print(match.span())


>>>
pam
4
7
(4, 7)
>>>

Search and Replace

One of the most important regular expressions is sub.

re.sub(<pattern>, <repl>, <string>, max=0)

pattern = the pattern you are matching.

repl = the thing you are replacing the matched pattern with.

string = The thing you are running the substitute on.

e.g:

import re

my_string = "My name is Renee. Hi Renee."
pattern = r"Renee"
new_string = re.sub(pattern, "Daniel", my_string)
print(new_string)


>>>
My name is Daniel. Hi Daniel.
>>>