Regular Expression Syntax

Introduction

A regular expression (regex) is a pattern that describes a set of strings. Python’s re module provides regex functionality.

Literal Characters

import re

pattern = r"hello"
text = "hello world"
print(re.findall(pattern, text))  # ['hello']

Metacharacters

Character	Meaning	Example
`.`	Any character except newline	`h.t` matches “hat”, “hot”
`^`	Start of string	`^hello` matches “hello world”
`$`	End of string	`world$` matches “hello world”
`*`	0 or more repetitions	`ab*c` matches “ac”, “abc”, “abbc”
`+`	1 or more repetitions	`ab+c` matches “abc”, “abbc”
`?`	0 or 1 repetition	`ab?c` matches “ac”, “abc”
`{n}`	Exactly n repetitions	`a{3}` matches “aaa”
`{n,}`	n or more repetitions	`a{2,}` matches “aa”, “aaa”
`{n,m}`	n to m repetitions	`a{2,4}` matches “aa”, “aaa”, “aaaa”
`[]`	Character class	`[aeiou]` matches any vowel
`	`	OR
`()`	Group	`(ab)+` matches “ab”, “abab”

Character Classes

# Pre-defined classes
# \d = digit (0-9)
# \w = word character (a-z, A-Z, 0-9, _)
# \s = whitespace (space, tab, newline)
# \b = word boundary

# Custom classes
pattern = r"[aeiou]"          # Any vowel
pattern = r"[0-9]"            # Any digit
pattern = r"[^0-9]"           # NOT a digit (negation)
pattern = r"[a-zA-Z]"         # Any letter
pattern = r"[A-Z][a-z]+"      # Capitalized word

text = "Hello World! 123"
print(re.findall(r"[A-Z][a-z]+", text))  # ['Hello', 'World']

Groups and Capturing

text = "John: 555-1234"

# Capturing groups
pattern = r"(\w+): (\d{3}-\d{4})"
match = re.search(pattern, text)
print(match.group(0))     # John: 555-1234 (full match)
print(match.group(1))     # John
print(match.group(2))     # 555-1234

# Named groups
pattern = r"(?P<name>\w+): (?P<phone>\d{3}-\d{4})"
print(match.group("name"))   # John

# Non-capturing groups
pattern = r"(?:\d{3}-)?\d{4}"  # ?: makes group non-capturing

Lookahead and Lookbehind

# Positive lookahead: x(?=y) — matches x only if followed by y
re.findall(r'\d+(?= dollars)', "100 dollars, 50 euros")
# ['100']

# Negative lookahead: x(?!y) — matches x only if NOT followed by y
re.findall(r'\d+(?! dollars)', "100 dollars, 50 euros")
# ['50']

# Positive lookbehind: (?<=y)x — matches x only if preceded by y
re.findall(r'(?<=\$)\d+', "Price: $100, Cost: 50")
# ['100']

Practice Exercises

Exercise 1: Write a regex that matches valid email addresses.

Exercise 2: Write a regex that matches phone numbers in format (123) 456-7890.

Exercise 3: Write a regex that extracts all URLs from a block of text.