regex.md

Regular Expressions (Reg ex)

Exact string match

['tugboat', 'tugboats']
Regex: 'tugboat'
Found: once, exact match.

Wild character match

'.' Will match any single character in a regex.
['tugboat', 'tugboats']
Regex: 'tugboat.'
Found: 'tugboats'

Match character if it exists, but do no require it

'?' Matches the character that appears before it, zero or one time.

['tugboat', 'tugboats']
Regex: 'tugboats?'
Found: 'tugboat', 'tugboats'

['tugboat', 'tugbots']
Regex: 'tugboa?ts?'
Found: 'tugboat', 'tugbots'

You can make a character set optional.

['toyboat', 'toyboats', 'toy boats', 'Toy boats', 'joy boats', 'toy-boats', 'toy-Boat']
Regex: [Ttj]oy[ -]?boats?
Note: The '?' before a character set makes a character set optional.
Found: 'toyboat', 'toyboats', 'toy boats', 'Toy boats', 'joy boats', 'toy-boats'
Excluded: 'toy-Boat'

Capital Case and lower case matching of characters

['tugboat', 'tugboats', 'toy boats', 'Toy boats']
Regex: '[Tt]oy ?boats?'
Found: 'toy boats', 'Toy boats'

'[]' No matter how many characters we put in the set, the set will only match the position of the character we put it in. It also makes no difference how you order the characters.

['tugboat', 'tugboats', 'toy boats', 'Toy boats', 'joy boats']
Regex: '[Ttj]oy[ -]?boats?'
Found: 'toy boats', 'Toy boats', 'joy boats'

Ranges of characters

If you want to find a range of characters such as 'A' thru 'Z'.

['toyboat', 'toyboats', 'toy boats', 'Toy boats', 'joy boats', 'toy-boats', 'toy-Boat']
Regex: '[A-Z]oy ?boats?'
Found: All upper case

If you want to find a range of characters such as 'A' thru 'Z' in both capital and lower case you can put the range of character right next to each other in the set.

['toyboat', 'toyboats', 'toy boats', 'Toy boats', 'joy boats', 'toy-boats', 'toy-Boat']
Regex: '[A-Za-z]oy[ -]?boats?'
Results: All

Regular Expression Wildcards

Matches more than one character in a string

[0-9] --> \d
[A- Za-z0-0] --> \w
[ \t\r\n\f] --> \s
Any Character --> .

Repeating Character Matches

Matched Pattern --> Character
Zero or More --> '*' --> None or all characters
One or more --> '+' --> Must have at least one character
Zero or one --? '?' --> May have none or one character.

Using '*'

['toy', 'toyboat', 'toycar']
Regex: 'toy\w*'
Result: Returns anything that starts with 'toy', has a letter or number after and has any number of characters after that. In this case, the whole array of strings is returned.

Using '+'

['toy', 'toyboat', 'toycar']
Regex: 'toy\w+'
Result: Returns anything that starts with 'toy' and has at least one character afterward.

Exact Number of Character Matches

Mattched Pattern --> Character
Three --> {3} --> Exactly three
Three or more --> {3, } --> Three or greater
Between 3 and 5 --> {3, 5}

Social security number example

\d{3}-\d{2}-\d{4}
000-23-7674
000-45-8792

Negated Characters

Matched Pattern --> Character
Negated character set --> [^]
Match and character except '@' --> [^@]
Match any character except '@' and '.' --> [^@.]

Opposite Characters

Character --> Opposite
\s White Space --> \S Not white space
\d Digit --> \D Not digit
\w Word --> \W not word
\s Whitespace --> \S Not whitespace

Alternation / Pipes

['toy boat', 'sail boat', 'steam boat', 'tug boat']
toy|sail|tug boat

Wildcard Group Matching

Matched Pattern --> Character
Matches 'subsub' --> (sub){2}
Macthes 'pat' and 'subpat' --> (sub)?pat

Matches three substraights

['toy boat', 'sail boat', 'steam  boat', 'tug boat']
Regex: (toy|sail|tug) boat
Match: Returns all except 'steam boat'

Partial Matches

Matched Pattern --> Character
Beginning of a string -- > '^'
End of a string --> '$'

Regex In Python

Opening a txt file to parse

"""

"""
import re

names_file = open("names.txt", encoding="utf-8")
data = names_file.read()
names_file.close()

print(re.match(r'Love', data))  # The 'r' in front of a string indicates that we are using a raw string; we can avoid using character escaping this way.
print(re.search(r'Kenneth', data))  # Use 'match' to find text at the beginning of a string. Use 'search' to find text anywhere in the string.