Regular expressions (or regex) are a powerful tool for processing and analyzing textual information. They are used to search, replace, and validate text according to specific patterns. Regular expressions are widely used in programming, data processing, search, and other fields.
Key concepts of regular expressions:
1. Characters:
* Literals: Literals represent themselves. For example, the regular expression abc matches the string "abc."
* Metacharacters: Special characters with specific meanings, such as a dot (.), which represents any character, or an asterisk (*), which represents 0 or more repetitions of the preceding character.
2. Character Classes:
* [ ]: Character classes allow specifying a set of characters. For example, [aeiou] matches any vowel.
* [^ ]: Negated character classes specify characters that should not match.
3. Repetition Quantifiers:
* {n}: Specifies the exact number of repetitions of the preceding element, e.g., a{3} matches the string "aaa."
* {n, m}: Specifies a range of repetitions, e.g., a{2,4} matches strings "aa," "aaa," and "aaaa."
* ?: Indicates that the preceding character or group can be optional or repeated once.
4. Special Sequences:
* \d: Digit (equivalent to [0-9]).
* \w: Alphanumeric character (equivalent to [a-zA-Z0-9_]).
* \s: Whitespace character.
* \b: Word boundary.
5. Alternation and Groups:
* |: Alternation allows choosing between specified alternatives, e.g., cat|dog matches either "cat" or "dog."
* ( ): Creates a group to combine characters.
6. Quantifiers:
* *?:, +, : Specify non-greedy, one or more, and zero or more quantifiers, respectively.
