Regex

Course: CSCI 1900

Regular Expression

• Pattern that can represent a variety of strings
• Tool for verifying information

Why?

• Validation
• Searching (such as grep)
• Identify valid credit card numbers
• Represent complex strings with single simpler* string

Set Correspondence

• The set of all distinct elements of the sequence
• is the set corresponding to the sequence abababababbababab

Basics

• Given a set
• would be the words that could be created with the characters, including non-valid words

Examples

Phone Number

• (123) 456-7890
  • Opening paren
  • 3 digits
  • Closing paren
  • Space
  • 3 digits
  • Hyphen
  • 4 digits
• \(\d{3}\) \d{3}-\d{4}

Email Address

• foo@bar.com
  • User
  • @
  • Domain
  • .
  • TLD
• \w+@\w+\.\w+$

Word Ending in ‘ing’

• Crying
  • A word
  • Literal “ing”
• \w+ing\b

Password

• SuperStr0ngPassword!
  • At least one letter → (?=.*[a-zA-Z])
  • At least one digit → (?=.*\d)
  • At least one special character → (?=.*[!@#$%^&*?])
  • At least 8 characters → [a-zA-Z\d!@#$%^&*?]{8,}
• ^(?=.*[a-zA-Z])(?=.*\d)(?=.*[!@#$%^&*?])[a-zA-Z\d!@#$%^&*?]{8,}$

Hex Color

• #ffffff or #fff
  • Hashtag
  • 6 or 3 characters, 0-9 or a-f
• ^#([a-fA-F\d]{6}|[a-fA-F\d]{3})

Python Code

import re
 
test_cases = [
	"(123) 456-7890",
	"foo@bar.com",
	"Crying",
	"MyPasswordIsVeryStr0ng!",
	"#abc"
]
 
patterns = {
	"Phone Number": r"\(\d{3}\) \d{3}-\d{4}",
	"Email Address": r"\w+@\w+\.\w+$",
	"Word ending in 'ing'": r"\w+ing\b",
	"Password": r"^(?=.*[a-zA-Z])(?=.*\d)(?=.*[!@#$%^&*?])[a-zA-Z\d!@#$%^&*?]{8,}$",
	"Hex Code": r"^#([a-fA-F\d]{6}|[a-fA-F\d]{3})"
}
 
for label, patterns in patterns:
	print(f"\nTesting {label}:")
	for case in test_cases:
		match = re.match(pattern, case)
		print(f"  {case}: {"✅" if match else "❌"}")