Regex in Python

Here is a sample file names.txt that we'll be using regex on.

Love, Kenneth kenneth@teamtreehouse.com (555) 555-5555 Teacher, Treehouse @kennethlove McFarland, Dave dave@teamtreehouse.com (555) 555-5554 Teacher, Treehouse Arthur, King king_arthur@camelot.co.uk King, Camelot Österberg, Sven-Erik governor@norrbotten.co.se Governor, Norrbotten @sverik , Tim tim@killerrabbit.com Enchanter, Killer Rabbit Cave Carson, Ryan ryan@teamtreehouse.com (555) 555-5543 CEO, Treehouse @ryancarson Doctor, The doctor+companion@tardis.co.uk Time Lord, Gallifrey Exampleson, Example me@example.com 555-555-5552 Example, Example Co. @example Obama, Barack president.44@us.gov 555 555-5551 President, United States of America @potus44 Chalkley, Andrew andrew@teamtreehouse.com (555) 555-5553 Teacher, Treehouse @chalkers Vader, Darth darth-vader@empire.gov (555) 555-4444 Sith Lord, Galactic Empire @darthvader Fernández de la Vega Sanz, María Teresa mtfvs@spain.gov First Deputy Prime Minister, Spanish Govt.

Let's create our python script file called regex.py. First things first, you need to import the regex expressions module.

import re

Then we'll open the names.txt file, read it and close it.

names_file = open("names.txt") data = names_file.read() names_file.close()

Now we have all the content of the names.txt file in a variable called data which we can now use to run regex on.

Let's try searching for a name

print(re.match(r'Love', data)) print(re.search(r'Kenneth', data))

r'string' - A raw string that makes writing regular expressions easier.

re.match(pattern, text, flags) - Tries to match a pattern against the beginning of the text.

re.search(pattern, text, flags) - Tries to match a pattern anywhere in the text. Returns the first match.

.match searches for a pattern in the beginning of $data and .search searches for a pattern anywhere in $data.

The result we would get would be something like this:

<_sre.SRE_Match object; span=(0, 4), match='Love'> <_sre.SRE_Match object; span=(6, 13), match='Kenneth'>

You can make the script easier to read by defining your search patterns as variables.

first_name = r'Love' last_name = r'Kenneth' print(re.match(first_name, data)) print(re.search(last_name, data))

Escape Characters

\w means any Unicode character including numbers, letters both uppercase and lowercase, special characters etc.

Let's try searching for a phone number now, say (555) 555-5555

number_pattern = r'(\d\d\d) \d\d\d-\d\d\d\d' print(re.search(number_pattern, data))

notice that we have escaped the parantheses ( ). By default, parantheses mean a regex group. To use parantheses in a match pattern, you need to escape them. The output would be:

<_sre.SRE_Match object; span=(40, 54), match='(555) 555-5555'>

Counts

Instead of doing \d\d\d\d\d for 5 numbers, we can specify counts like {3} for somethnig that occurs 3 times and {,3} for something that occurs 0 to 3 times.