Regular expressions

Lesson 37 of 62, about 4 min

A regular expression (regex) is a pattern that describes a set of strings. With the re module you can check whether text matches a pattern, find every match, or pull out the parts you care about. Regex is compact and powerful, and a little goes a long way.

Searching for a pattern

python

import re

text = "Order 4521 shipped on 15-09-2026"
m = re.search(r"\d+", text)
print(m.group())
print(m.start(), m.end())

re.search finds the first match anywhere in the string and returns a match object, or None if nothing matches. Always write patterns as raw strings (r"...") so backslashes are passed through untouched.

The building blocks

Pattern	Matches
`.`	Any character except newline
`\d`	A digit
`\w`	A letter, digit or underscore
`\s`	Whitespace
`[abc]`	One of a, b or c
`[a-z]`	One lowercase letter
`[^0-9]`	Anything that is not a digit
`^`	Start of string
`$`	End of string
`a\|b`	a or b

Quantifiers say how many times the thing before them may repeat:

Quantifier	Meaning
`*`	0 or more
`+`	1 or more
`?`	0 or 1
`{3}`	Exactly 3
`{2,5}`	2 to 5

python

import re

print(re.search(r"^\d{4}$", "2026") is not None)
print(re.search(r"^\d{4}$", "20261") is not None)
print(re.search(r"colou?r", "color").group())

Finding all matches

python

import re

text = "Call 98765 43210 or 91234 56789"
print(re.findall(r"\d{5} \d{5}", text))

Groups

Parentheses capture parts of the match so you can pull them out separately.

python

import re

m = re.search(r"(\d{2})-(\d{2})-(\d{4})", "Due on 15-09-2026")
print(m.group(0))
print(m.group(1), m.group(2), m.group(3))
print(m.groups())

Named groups make this readable:

python

import re

pattern = r"(?P<day>\d{2})-(?P<month>\d{2})-(?P<year>\d{4})"
m = re.search(pattern, "Due on 15-09-2026")
print(m.group("year"))
print(m.groupdict())

Replacing

python

import re

print(re.sub(r"\s+", " ", "too    many   spaces"))
print(re.sub(r"(\w+)@(\w+)\.com", r"\1 at \2", "mail sara@indus.com now"))

Splitting

python

import re
print(re.split(r"[,;]\s*", "a, b;c ,d"))

Compiling a pattern

If you use a pattern many times, compile it once.

python

import re

email = re.compile(r"^[\w.+-]+@[\w-]+\.[\w.]+$")
for candidate in ["priya@example.com", "not an email", "a.b+c@dept.uni.in"]:
    print(candidate, bool(email.match(candidate)))

match checks only at the start of the string; search looks anywhere; fullmatch requires the whole string to match.

Flags

python

import re
print(re.findall(r"python", "Python is python", re.IGNORECASE))

Greedy versus lazy

.* grabs as much as it can. Add ? to make it stop at the first opportunity.

python

import re
html = "<b>bold</b> and <i>italic</i>"
print(re.findall(r"<.*>", html))
print(re.findall(r"<.*?>", html))

Practice

Extract all hashtags from "Learning #python and #ml this week #100DaysOfCode".
Validate Indian mobile numbers: ten digits starting with 6 to 9.
Replace every date in dd-mm-yyyy format with yyyy-mm-dd using groups.