Regular Expressions

Regex

Regular expressions are:

• patterns used to match and manipulate text strings

Basic Patterns

• Literal characters match themselves (“cat” matches “cat”)
• Special characters have specific meanings:
  • . (dot) - matches any single character except newline
  • * matches 0 or more of the previous character
  • + matches 1 or more of the previous character
  • ? - matches 0 or 1 of the previous character
  • ^ - matches start of line
  • $ - matches end of line

Other syntax

• \d - matches any digit (0-9)
• \w - matches any word character (a-z, A-Z, 0-9, _)
• \s - matches any whitespace character (space, tab, newline)
• [] used to indicate a set of characters.
• {m} matches m times
• {m,n} matches m to n times

e.g., lower case letters: [a-z], upper case letters: [A-Z]

Examples

[abc] - matches any single character from the set (a, b, or c)

[^abc] - matches any single character NOT in the set

Python examples

Email validation pattern:

pattern = r'^[\w\.-]+@[\w\.-]+\.\w+$'

Phone number pattern (US format):

pattern = r'\d{3}-\d{3}-\d{4}'