Regular Expressions Basics

Study Notes

Regular Expressions: Metacharacters and Special Sequences

Metacharacters serve specific functions in regex, often with special meanings.
Basic Metacharacters:
- \w: Matches any word character (alphanumeric + underscore).
- \W: Matches any non-word character.
- \d: Matches any digit (equivalent to [0-9]).
- \D: Matches any non-digit character.
- \s: Matches any whitespace character (space, tab, etc.).
- \S: Matches any non-whitespace character.
- \b: Matches at the beginning or end of a word.
- \B: Matches anywhere except at the word boundaries.
- Control characters include \0, \n, \r, \t, and others for specific character matches.

Special Sequences in Regex

\A: Asserts position at the start of a string.
\Z: Asserts position at the end of a string.
\xxx: Matches character specified by octal number.
\xdd: Matches character specified by hexadecimal number.
\udddd: Matches Unicode character specified by hexadecimal.

Quantifiers

Quantifiers define how many times a character or group appears:
- n+: At least one occurrence of n.
- n*: Zero or more occurrences of n.
- n?: Zero or one occurrence of n.
- n{X}: Exactly X occurrences of n.
- n{X,Y}: Between X and Y occurrences of n.
- n{X,}: At least X occurrences of n.
- ^n: Line starts with n.
- n$: Line ends with n.

Common Patterns with Metacharacters

* (Star): Matches zero or more occurrences of the preceding element.
+ (Plus): Matches one or more occurrences of the preceding element.
? (Question Mark): Matches zero or one occurrence of the preceding element.
{} (Braces): Specify exact number of repetitions.

Regex Modifiers

Modifiers change how the pattern behaves:
- g: Global search (find all matches).
- i: Case-insensitive search.
- m: Multiline search.
- u: Unicode search.

Example Patterns

Email Matching Pattern:
- Begins with /, checks for starting line with characters.
- Uses character classes and quantifiers.
- '@' character is matched literally, followed by domain patterns.
URL Matching Pattern:
- Starts with /, uses ^ for line start.
- Matches http or https, followed by proper formatting with characters.

Additional Notes

Regular expressions require precise syntax and structure for effective pattern matching.
Understanding metacharacters and quantifiers is crucial for writing and interpreting regex patterns.

Description

This quiz covers the fundamental concepts of regular expressions, including character categories such as word characters, digits, and whitespace. Test your understanding of how to identify and classify different types of characters using regex syntax.