Regex Flags

Regex Special Characters

These notes cover special characters used in Regular Expressions within Python. Remember to import the re module before using any regex functions. import re

Regex Metacharacters These are symbols with special meanings in regex.

• . (Dot): Matches any single character except a newline.

• ^ (Caret): Matches the beginning of a string.

• $ (Dollar): Matches the end of a string.

• * (Asterisk): Matches zero or more occurrences of the preceding character or group.

• + (Plus): Matches one or more occurrences of the preceding character or group.

• ? (Question Mark): Matches zero or one occurrence of the preceding character or group.

• 1 (Square Brackets): Defines a character set. e.g., [abc]] matches ‘a’, ‘b’, or ‘c’. Ranges are possible: [a-z]] matches any lowercase letter. [^abc]] negates the set (matches anything except a, b, or c).

• () (Parentheses): Creates a capturing group. Used for extracting specific parts of a matched string. Also useful for grouping parts of the regex for applying quantifiers.

• \| (Vertical Bar): Acts as an “or” operator. e.g., cat|dog matches either “cat” or “dog”.

• \ (Backslash): Escapes special characters. e.g., \. matches a literal dot, \( matches a literal opening parenthesis. It’s also used for special escape Sequences like \d (digit), \w (alphanumeric), \s (whitespace).

Regex Quantifiers These characters control how many times a preceding element should appear. (Already mentioned some above, but details here are crucial)

• * (Asterisk): Zero or more occurrences.

• + (Plus): One or more occurrences.

• ? (Question Mark): Zero or one occurrence.

• {n}: Exactly n occurrences.

• {n,}: n or more occurrences.

• {n,m}: Between n and m occurrences (inclusive).

Example:

import re

text = "The quick brown fox jumps over the lazy fox."
pattern = r"fox"  # Matches the literal string "fox"

matches = re.findall(pattern, text)
print(matches)  # Output: ['fox', 'fox']]

pattern = r".ox" # Matches any character followed by "ox"
matches = re.findall(pattern, text)
print(matches) # Output: ['fox', 'fox']]

pattern = r"f.x" # Matches 'f' followed by any character followed by 'x'
matches = re.findall(pattern, text)
print(matches) # Output: ['fox', 'fox']]

pattern = r"The.*fox" # Matches 'The' followed by any characters (except newline) followed by 'fox'
matches = re.findall(pattern, text)
print(matches) # Output: ['The quick brown fox']]

pattern = r"\bf\w+\b" #Matches whole words starting with 'f'
matches = re.findall(pattern,text)
print(matches) # Output: ['fox', 'fox']]

Regex Escape Sequences Details on \d, \w, \s, etc.

Python re Module Functions like re.findall, re.search, re.match, re.sub, etc.