Python RegEx

1. What is RegEx

Regular Expressions (RegEx) provide a powerful way to search, match, and manipulate text patterns.

import re

text = "Python is powerful"
match = re.search("Python", text)
print(match.group())  # Python

The re module enables pattern-based text processing.

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2. Using re.match()

import re

text = "Python Programming"
result = re.match("Python", text)

print(result.group())  # Python

match() checks for a pattern at the start of the string only.

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3. Using re.search()

import re

text = "Learn Python Programming"
result = re.search("Python", text)

print(result.group())  # Python

search() scans the entire string for the first match.

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4. Using re.findall()

import re

text = "The rain in Spain"
matches = re.findall("ai", text)

print(matches)  # ['ai', 'ai']

Returns all matches as a list.

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5. Using re.sub() (Replace Text)

import re

text = "I love Java"
new_text = re.sub("Java", "Python", text)

print(new_text)  # I love Python

Replaces matched patterns with new content.

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6. Using Character Classes

import re

text = "abc123"
result = re.findall("[a-z]", text)

print(result)  # ['a', 'b', 'c']

Common patterns:

• [a-z] → lowercase letters

• [A-Z] → uppercase letters

• [0-9] → digits

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7. Metacharacters and Special Sequences

import re

text = "Email: [e-mail]"
pattern = r"\w+@\w+\.\w+"

match = re.search(pattern, text)
print(match.group())

Common sequences:

• \d → digit

• \w → alphanumeric

• \s → whitespace

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8. Using Quantifiers

import re

text = "Helloooo"
pattern = r"o+"

print(re.findall(pattern, text))  # ['oooo']

Quantifiers:

• * → 0 or more

• + → 1 or more

• ? → 0 or 1

• {n} → exact count

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9. Grouping with Parentheses

import re

text = "John Doe"
pattern = r"(John) (Doe)"

match = re.search(pattern, text)
print(match.groups())  # ('John', 'Doe')

Groups allow segmented extraction of patterns.

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10. Real-World RegEx Example (Phone Validation)

import re

def validate_phone(phone):
    pattern = r"^\+?\d{10,13}$"
    return bool(re.match(pattern, phone))

print(validate_phone("[phone]"))  # True
print(validate_phone("12345"))          # False

Used for input validation and sanitization in production systems.

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Common RegEx Patterns Reference

Pattern	Meaning
\d	Digit
\D	Non-digit
\w	Alphanumeric
\W	Non-alphanumeric
^	Start of string
$	End of string
.	Any character
[]	Character set
()	Grouping

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