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Python Regular Expressions in Python Introduction to Regular Expressions Compiling and Loops

igsm '
igsm '
10,440 Points

Boolean with re?

Hey. Can you help please with the following. Using RE, I want to check a string, which is meant to be a passwords. True if it contains >= 10 chars, at least one lower letter, one capital letter and one number; else False. How would a code look like. Thanks.

e.g. two cases:

True = 'bAse730onE4'

False = 'saaaa90'

re.match('[a-zA-Z0-9]+', password)

3 Answers

Chris Freeman
Chris Freeman
Treehouse Moderator 68,404 Points

Because you are looking for the AND of multiple conditions there isn't a short single concise regexp that will solve your needs because your conditions are not order dependent. That is, you would have to check for all combinations of a condition followed by all others. For example lowercase followed by uppercase followed by number and being 10 characters long, plus the other 5 combinations, ORed together in a single regexp.

The other option is to create the 3 search patterns and check that they each match plush a length check.

patterns = []
patterns.append(re.compile(r'[a-z]+')) #lowercase
patterns.append(re.compile(r'[A-Z]+')) #uppercase
patterns.append(re.compile(r'[0-9]+') )# number
password_good = True
if len(password_string) < 10:
    password_good = False
    for pat in patterns:
        if not pat.search(password_string):
            # pattern not found
            password_good = False

You could include the length check as a regexp:


I'll work on the combined uber-regexp to show it's complications and ammend my answer.

EDIT: Here is a single regexp pattern to match against. Yeah, it's not pretty:

In [101]: patx = re.compile(r'''
''', re.X)

In [102]: patx.search("")

In [103]: patx.search("a")

In [104]: patx.search("aA1")
Out[104]: <_sre.SRE_Match object; span=(0, 3), match='aA1'>

Now you can use:

if not len(password) > 10 and patx.search(password)
    print("Bad Password")
Chris Freeman
Chris Freeman
Treehouse Moderator 68,404 Points

Added uber-regexp to answer.

If switching to re.match instead of search, use the patx as above:

re.match(patx, password)

If you choose to switch to re.search, you can remove the leading and trailing \w* from each line in the patx regexp.

This works, too:

import re

def check_password(string):
    return (re.search('[a-z]+', string) and re.search('[A-Z]+', string) 
            and re.search('[0-9]+', string) and len(string) > 9)

Technically, it actually returns None when the password is bad, but that's fine in a lot of applications. Easy modification if the False return is important:

import re

def check_password(string):
    if (re.search('[a-z]+', string) and re.search('[A-Z]+', string) 
            and re.search('[0-9]+', string) and len(string) > 9):
        return True
        return False

Hi Igor, re.search and re.match return match objects. If the item isn't found, it returns None (the special value). re.match checks for a match at the beginning of the string, whereas re.search checks anywhere in the string. You can use the boolean operator "and" to check multiple conditions--for example:

def check_two(string):
   return re.search(\w+) and re.search(\d+)

returns True if the string contains at least one word character ([a-zA-Z0-9_]) and at least one decimal digit. You can get more specific by using a set definition, like [a-z]+.

igsm '
igsm '
10,440 Points

Thank you. Really helpful!