Regular Expression

Regular Expression (RegEx) is a special sequence of characters used to search for patterns in strings.

The re module is used for regular expressions in Python.

Common Functions in the re Module:

• fullmatch(): Returns a match object if the entire string matches the pattern.
• search(): Returns a match object for the first occurrence of the pattern anywhere in the string.
• findall(): Returns a list of all matches of the pattern in the string.
• split(): Splits the string at each match of the pattern.
• sub(): Replaces all matches with a specified string.

Creating a Regular Expression

A regular expression is created using a mix of characters, special sequences, and sets.

Characters

• []: Represents a set of characters or numbers.
• \: Represents a special sequence (e.g., \s, \d, \w).
• ^: Represents patterns present at the beginning of a string.
• *: Represents 0 or more occurrences. Example: [0-9]*
• +: Represents 1 or more occurrences. Example: [0-9]+
• {}: Specifies the number of occurrences of a pattern. Example: [a-z]{2}
• |: Represents "this or that" in the pattern.

Rules of Regex:

• [abc]: Matches either a, b, or c.
• [^abc]: Matches anything except a, b, or c.
• [a-z]: Matches any lowercase letter (a to z).
• [A-Z]: Matches any uppercase letter (A to Z).
• [a-zA-Z]: Matches both lowercase and uppercase letters.
• [0-9]: Matches any digit (0 to 9).
• [a-zA-Z0-9]: Matches lowercase, uppercase letters, or digits.

Special Sequences:

• \s: Matches a space.
• \d: Matches a digit.
• \D: Matches anything except a digit.
• \w: Matches a word character (alphanumeric).

Quantifiers:

• [a-z]+: Matches one or more lowercase letters.
• [a-z]*: Matches zero or more lowercase letters.
• [a]?: Matches if a is included or not included (0 or 1 occurrence).
• [a]{n}: Matches exactly n occurrences of a. Example: [a-z]{2}
• [a]{n,k}: Matches at least n and at most k occurrences of a.