re module

re.compile(pattern, flags=0)

The re module provides regular expression operations. It supports pattern matching, searching, replacing, and splitting text using regular expression patterns.

Syntax

import re

Core Functions

re.compile()

Compiles a regular expression pattern into a regex object for reuse.

import re

pattern = re.compile(r"\d{3}-\d{4}")
result = pattern.search("Call me at 555-1234 tomorrow")
print(result.group())
# 555-1234

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern to compile
flags	int	0	Bitwise flags that modify pattern behavior

re.search()

Searches for the first match of a pattern in a string.

text = "The price is $19.99"
match = re.search(r"\$([0-9]+\.[0-9]+)", text)
if match:
    print(match.group(1))
# 19.99

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern
string	str	Required	The string to search
flags	int	0	Bitwise flags that modify pattern behavior

re.match()

Matches only at the beginning of the string.

text = "hello world"
match = re.match(r"hello", text)
print(match.group() if match else "No match")
# hello

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern
string	str	Required	The string to search
flags	int	0	Bitwise flags that modify pattern behavior

re.findall()

Returns all non-overlapping matches as a list.

text = "There are 3 cats and 5 dogs"
numbers = re.findall(r"\d+", text)
print(numbers)
# [3, 5]

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern
string	str	Required	The string to search
flags	int	0	Bitwise flags that modify pattern behavior

re.finditer()

Returns an iterator of all match objects.

text = "Split this: one, two, three"
for match in re.finditer(r"\w+", text):
    print(f"Found {match.group()} at {match.start()}")
# Found Split at 0
# Found this at 6
# Found one at 12
# Found two at 16
# Found three at 21

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern
string	str	Required	The string to search
flags	int	0	Bitwise flags that modify pattern behavior

re.sub()

Replaces all matches with a replacement string.

text = "The cat sat on the mat"
result = re.sub(r"cat", "dog", text)
print(result)
# The dog sat on the mat

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern
repl	str	Required	The replacement string (can include backreferences)
string	str	Required	The string to search
count	int	0	Maximum number of replacements (0 = all)
flags	int	0	Bitwise flags that modify pattern behavior

re.subn()

Like sub() but returns a tuple with the replacement count.

text = "aaa bbb aaa ccc"
result, count = re.subn(r"aaa", "xxx", text)
print(f"Result: {result}, Count: {count}")
# Result: xxx bbb xxx ccc, Count: 2

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern
repl	str	Required	The replacement string
string	str	Required	The string to search
count	int	0	Maximum number of replacements
flags	int	0	Bitwise flags that modify pattern behavior

re.split()

Splits the string by matches of the pattern.

text = "apple, banana; orange: grape"
parts = re.split(r"[,;: ]+", text)
print(parts)
# [apple, banana, orange, grape]

Parameters

Parameter	Type	Default	Description
pattern	str	Required	The regular expression pattern
string	str	Required	The string to split
maxsplit	int	0	Maximum number of splits
flags	int	0	Bitwise flags that modify pattern behavior

Match Object Methods

The methods below are called on Match objects returned by functions like re.search(), re.match(), and re.finditer().

match.group()

Returns the matched string or a specific captured group.

pattern = re.compile(r"(\w+)@(\w+)\.(\w+)")
match = pattern.search("user@example.com")
print(match.group())      # user@example.com
print(match.group(1))     # user
print(match.group(2))     # example
print(match.group(3))     # com

Parameters

Parameter	Type	Default	Description
group	int or str	0	The group number (0 = entire match) or named group string

match.groups()

Returns all captured groups as a tuple.

pattern = re.compile(r"(\w+)-(\d+)")
match = pattern.search("abc-123")
print(match.groups())
# (abc, 123)

Common Patterns

Email validation

email = "user@example.com"
pattern = r"^[\w.-]+@[\w.-]+\.\w+$"
if re.match(pattern, email):
    print("Valid email")
# Valid email

Phone number extraction

text = "Call 555-123-4567 or 555.987.6543"
phones = re.findall(r"\d{3}[-. ]\d{3}[-. ]\d{4}", text)
print(phones)
# [555-123-4567, 555.987.6543]

Named groups

pattern = r"(?P<name>\w+) (?P<age>\d+)"
text = "John 30"
match = re.search(pattern, text)
print(match.group("name"))
# John
print(match.group("age"))
# 30

Pattern Flags

Use flags to modify regex behavior:

text = "Hello\nWorld"

# Case-insensitive matching
print(re.search(r"hello", text, re.I).group())
# Hello

# Multiline mode
print(re.search(r"^World", text, re.M).group())
# World

# Dot matches newlines
print(re.search(r".+", text, re.DOTALL).group())
# Hello\nWorld

Errors

Error	When it occurs
re.error	Invalid regex pattern syntax
None	re.search()/re.match() returns None when no match found

See Also

• string
• textwrap
• difflib