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Overview

Regex Practice

September 27, 2021
1 min read
nlp
python
KOENIT
import re
password_checker = re.compile("^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*])[A-Za-z\d!@#$%^&*]{8,30}$"  )
mat = re.search(password_checker, password)

I tend to only use the patterns I already know, so encountering something new like this freezes me up. I took this as a chance to practice and analyze it. ref: Blog, COGNEX, Mozilla

  • ^: start of a line
  • $: end of a line
  • |: and
  • \t: tab
  • \n: newline
  • .: any character
  • []: match any character inside the brackets, regardless of order
    • e.g., [a-ZA-Z]
  • [^]: match anything except the characters inside the brackets
    • e.g., [^a-Z]: search excluding lowercase alphabet
  • *: zero or more of the preceding character
    • Repeats the character before ’*’.
    • e.g., ‘bo*’ matches ‘b’ and also ‘booooo’.
  • ?: zero or one of the preceding character
  • x(?=y): called a lookahead. Matches x only if followed by y.
  • \d: same as 0-9. Matches digits.

Application

import re
password_checker = re.compile("^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*])[A-Za-z\d!@#$%^&*]{8,30}$"  )
mat = re.search(password_checker, password)

The regex above finds strings that satisfy the following conditions:

  • 8 to 30 characters long
  • At least one uppercase and one lowercase English letter
  • At least one digit
  • At least one special character (!@#$%^&*)

Condition checks

(?=.[a-z])(?=.[A-Z])(?=.\d)(?=.[!@#$%^&*]) checks each condition one by one. If any of the 4 conditions is false, all 4 groups evaluate to false.

Looking at the blog posts below, it’s essentially chaining multiple conditions like an AND operation.

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