Regular Expressions using Python

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I am writing a function using regular expressions for emails. I think I write the pattern correctly, however, I couldn't find out why example 2 'what-do-you-know+not-much@gmail.com' failed to be detected while example 1 worked successfully?

def parse_email(s):
    try:
        pattern = re.compile(r'\b([a-zA-Z])([\w.-_+]+)@([\w.-]+)([a-zA-Z])\b')
        matches = pattern.finditer(s)
        for match in matches:
            print(match.group(0))
            return (match.group(1)+match.group(2), match.group(3)+match.group(4))
    except AttributeError:
        #print('here')
        raise ValueError


print(parse_email('JKRowling@Huge-Books.org'))
print(parse_email('what-do-you-know+not-much@gmail.com'))

Results:

JKRowling@Huge-Books.org
('JKRowling', 'Huge-Books.org')

much@gmail.com
('much', 'gmail.com')
python regex
2 Answers

From re docs:

Ranges of characters can be indicated by giving two characters and separating them by a '-', for example [a-z] will match any lowercase ASCII letter, [0-5][0-9] will match all the two-digits numbers from 00 to 59, and [0-9A-Fa-f] will match any hexadecimal digit. If - is escaped (e.g. [a-z]) or if it’s placed as the first or last character (e.g. [-a] or [a-]), it will match a literal '-'. [emphasis added]

It looks like you are trying to match a literal -, so place it as the first character of the range, e.g. [-xxx]:

pattern = re.compile(r'\b([a-zA-Z])([-\w._+]+)@([-\w.]+)([a-zA-Z])\b')

Test:

>>> import re
>>> pat = r"\b([a-zA-Z])([-\w._+]+)@([-\w.]+)([a-zA-Z])\b"
>>> old_pattern = re.compile(r'\b([a-zA-Z])([\w.-_+]+)@([\w.-]+)([a-zA-Z])\b')
>>> new_pattern = re.compile(r'\b([a-zA-Z])([-\w._+]+)@([-\w.]+)([a-zA-Z])\b')
>>> old_pattern.search('what-do-you-know+not-much@gmail.com')
<re.Match object; span=(21, 35), match='much@gmail.com'>
>>> new_pattern.search('what-do-you-know+not-much@gmail.com')
<re.Match object; span=(0, 35), match='what-do-you-know+not-much@gmail.com'>
user_79545040

Welcome to the wonderful world of regular expressions, where the tiniest of changes can result in totally unexpected outcomes.

To start, let's analyze the regex pattern you have:

r'\b([a-zA-Z])([\w.-_+]+)@([\w.-]+)([a-zA-Z])\b'

  • \b is a correct choice, since you want items which are there own word. Be careful though, since this won't include the beginning or end of a string.
  • ([a-zA-Z]) if your first capture group. You can replace with the simpler ([A-z])
  • ([\w.-_+]+) is your second capture group. It will capture:
    • \w any word character (redundant)
    • . will not necessarily capture the period character, instead, is capturing "any" character
    • - will not capture the dash character, instead, will capture a range of characters
    • _ will indeed capture underscore characters – but in this case, it's being referenced as the end of a range
    • + will not capture plus characters, rather, will get "1 or more" characters from a group or range.

... I'll stop here, since the rest is more or less similar...

You'll want to replace your regex with the following:

r'\b([A-z0-9\-\+]+@[A-z\-\+]+\.[A-z]{3})\b'

  • There is only one capture group, since we want entire email addresses.
  • Email addresses (here) are allowed to contain:
    • Before the at symbol: [A-z0-9\-\+]+ all alpha-numeric characters as well as '-' and '+' characters (as denoted by the escaped characters \- and \+
    • Following the at symbol, a domain name [A-z\-\+] with alpha characters and escaped chars
    • Followed by a domain extension \.[A-z]{3} Ex: .org

Next, you can refactor your code to the following:

import re

pattern = re.compile(r'\b([A-z0-9\-\+]+@[A-z\-\+]+\.[A-z]{3})\b')
match = pattern.search(s)

if match:
   email = match.group()
else:
   email = None
user_105219590
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