i dont get \b and ^$ in regex python

Question

when i click check work, i get this message "Got "@teamtreehouse"."

i have specified my regex to be in between word boundaries (so the search should return something in between two non alphanumeric characters). to the left of '@teamtreehouse', there is a alphanumeric character. so how can '@teamtreehouse' be returned if to the left of the @ sign there is an alphanumeric character. only words with a space or non alphanumeric character to the left and right of the @ sign should be returned.

so looking at the first line of 'string', only '@kennethlove' should be returned because i have set a boundary and on both sides of the word there is whitespace. 'kenneth+challenge*@teamtreehouse.com*' (bolded part) should NOT be returned because i set a boundary to the left of the @ sign yet it does so i know this was alot of text but i dont understand

emails.py

import re

string = '''Love, Kenneth, kenneth+challenge@teamtreehouse.com, 555-555-5555, @kennethlove
Chalkley, Andrew, andrew@teamtreehouse.co.uk, 555-555-5556, @chalkers
McFarland, Dave, dave.mcfarland@teamtreehouse.com, 555-555-5557, @davemcfarland
Kesten, Joy, joy@teamtreehouse.com, 555-555-5558, @joykesten'''

contacts = re.search(r'''
        (?P<email>[-\w\d.+?]+@[-\w\d.]+),\s
        (?P<phone>\d{3}[-\.\s]\d{3}[-\.\s]?\d{4})
''', string, re.M | re.X)

#task 2

twitters = re.search(r'\b@\w+\b', string, re.M)

and also i dont know when to use ^ or $.

im aware that the following works:

twitters = re.search(r'@\w+$', string, re.M)

but why do i need to use $ in order for the code to work?

Answer 1 · 2016-02-08T05:45:25Z

February 8, 2016 5:45am

The '\b' represents a word boundary, that is a non-word character must occur in this position. In the twitter search, the @-sign is a non-word character and is effectively the same as '@\b'. Putting the \b before the @ causes the search to look for a word character preceding the \b (otherwise it wouldn't be word boundary). Modifying your pattern for this gives:

twitters = re.search(r'@\b\w+\b', string, re.M)

This issue now is that while the second \b marks the end of the twitter name, it also match the end of the email domain name since the domain ends in a non-word-char period before the Top-Level domain "com", etc.

Since an re.search() can be match anywhere with in the string, there are two other ways to anchor a pattern: using a ^ and $. These anchor the pattern to the beginning or end of a line. Since the twitter name ends the line, adding a '$' at the end of the pattern says after the set of word characters the End-of-Line must follow.

twitters = re.search(r'@\b\w+\b$', string, re.M)

Now since '@ and '$' are effectively non-word characters, they make the \b usage redundant and the \b can be removed:

twitters = re.search(r'@\w+$', string, re.M)

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sibal gesekki

sibal gesekki

i dont get \b and ^$ in regex python

1 Answer

Chris Freeman

Chris Freeman

sibal gesekki

sibal gesekki