20-Regex

Regex

There are a ton of regex symbols. You can test them out in any regex editor.

They can vary a little bit based on what language you're working with.

Rules

Characters

• . can match everything
• \ is used to escape a string
• \. will only match a .
• ab will only match ab
• \d matches digit 0-9
• \w matches letter, digit, or underscore

There are a bunch of cheatsheets online to refer to these rules.

The capital version of these rules reverse the rule

• \W gives anything that is not letter, digit or underscore
• \D matches anything that is not a number form 0-9

Quantifiers

They specify how many times something should occur in the text

• + matches one or more characters
• {x} matches exactly x times
• {3,5} 3 to 5 times
• {4,} 4 or more times
• * zero or more times
• ? once or none (optional)

Character Classes

These are group or sets of different characters

• [aeiou] matches any one of the 5; if we just do aeiou then it'll match the exact string with these 5 letters
• [aeiou]{2} finds characters whenever any 2 vowels are together
• [A-Z] refers to all uppercase letters
• [A-Z]{4,} matches all words with 4 or more uppercased letters
• [^k] matches everything that isn't k

Anchors or Boundaries

• ^ start of string or line
• $ end of string or line
• \b word boundary

Logical Operator |

Allows us to write optionals strings, for example Mr. | Ms. | Mrs.

• The pipe character selects b/w the strings before and after it
• Sometimes they need to be wrapped inside parenthesis to avoid mis-matches

Regex in Python

# import re module
import re

# define a basic phone number regex
pattern = re.compile(r'\d{3} \d{3}-\d{4}')
# here `r` is used to indicate that the string is a raw string so we don't need to escape the backslashes

# search a string with our regex
result = pattern.search('My number is 415 555-4242.')

result2 = pattern.findall('My number is 415 555-4242 and 888 000-3333.')

print(result.group()) # 415 555-4242
print(result2) # ['415 555-4242', '888 000-3333']