Regular Expression Patterns

Regular expressions are powerful tools for pattern matching in text. They help identify specific characters, groups, and structures, making them essential in programming. Understanding these patterns enhances your coding skills and problem-solving abilities in various programming languages.

1. Basic characters and literals

   • Represent the simplest form of matching in regular expressions.
   • Match exact characters, such as letters, digits, and symbols.
   • Case-sensitive by default, meaning 'a' and 'A' are different.

2. Wildcards (.)

   • Matches any single character except for newline characters.
   • Useful for flexible pattern matching where the exact character is unknown.
   • Can be combined with other patterns for more complex searches.

3. Character classes []

   • Define a set of characters to match any one character from the set.
   • Can include ranges (e.g., [a-z] matches any lowercase letter).
   • Useful for matching specific groups of characters without listing them all.

4. Negated character classes [^]

   • Matches any character not listed in the brackets.
   • Useful for excluding specific characters from a match.
   • For example, [^0-9] matches any character that is not a digit.

5. Quantifiers (*, +, ?, {n}, {n,}, {n,m})

   • Control how many times the preceding element can occur.
   • * matches zero or more times, + matches one or more times.
   • {n} matches exactly n times, {n,} matches n or more times, and {n,m} matches between n and m times.

6. Anchors (^ and $)

   • ^ asserts the position at the start of a line or string.
   • $ asserts the position at the end of a line or string.
   • Useful for ensuring patterns match at specific locations in the text.

7. Alternation (|)

   • Acts like a logical OR, allowing for multiple options in a pattern.
   • For example, cat|dog matches either "cat" or "dog".
   • Can be used to combine different patterns into a single expression.

8. Grouping and capturing ()

   • Groups multiple tokens together to treat them as a single unit.
   • Captures the matched content for later use or reference.
   • Useful for applying quantifiers to entire groups or extracting specific parts of a match.

9. Escaping special characters ()

   • Allows for the inclusion of special characters in a pattern without invoking their special meaning.
   • For example, \. matches a literal period instead of any character.
   • Essential for matching characters like *, +, ?, (, ), [, ], {, }, and \ itself.

10. Shorthand character classes (\d, \w, \s)

    • \d matches any digit (equivalent to [0-9]).
    • \w matches any word character (letters, digits, or underscores).
    • \s matches any whitespace character (spaces, tabs, line breaks).