Regular Expressions

Regex Metacharacters

These characters have special meaning within Regular Expressions and are not treated literally. They modify the behavior of other characters or patterns. It’s crucial to understand them for effective regex use.

List of Common Metacharacters:

• . (dot): Matches any single character except a newline.
• ^: Matches the beginning of a string.
• $: Matches the end of a string.
• *: Matches zero or more occurrences of the preceding character or group.
• +: Matches one or more occurrences of the preceding character or group.
• ?: Matches zero or one occurrence of the preceding character or group.
• 1: Defines a character set. Matches any single character within the brackets. [a-z]] matches any lowercase letter. [0-9]] matches any digit.
• [^]]: Negates a character set. Matches any character not within the brackets.
• (): Creates a capturing group. Allows you to extract specific parts of a matched string. Also used for grouping in quantifiers (e.g., (ab)+).
• |: Acts as an “or” operator. Matches either the expression before or after the pipe. cat|dog matches either “cat” or “dog”.
• \: Escapes a metacharacter, treating it as a literal character. \. matches a literal dot. \* matches a literal asterisk.

Examples:

import re

text = "The quick brown fox jumps over the lazy dog."

# Matches any character followed by a 'o'
print(re.findall(r".o", text))  # Output: ['Th', 'o', 'o']]

# Matches the string only if it starts with 'The'
print(re.findall(r"^The", text)) # Output: ['The']]

# Matches the string only if it ends with '.'
print(re.findall(r"\. $ ", text)) # Output: ['.']]

# Matches 'o' followed by zero or more 'x' characters
print(re.findall(r"ox*", text)) # Output: ['o', 'ox']]

# Matches one or more occurrences of 'l' followed by 'o'
print(re.findall(r"l+o", text)) # Output: ['lo']]

# Matches either 'fox' or 'dog'
print(re.findall(r"fox|dog", text)) # Output: ['fox', 'dog']]

#Using capturing group
print(re.findall(r"(quick) (brown)", text)) #Output:[('quick', 'brown')]]

Regex Character Sets
Regex Quantifiers Regex Grouping and Capturing Regex Special Sequences