Regex Notes

1. Character Classes []

• Definition: Matches any one character from a specified set.

• Syntax: [abc] matches a, b, or c.

• Ranges: [0-9] matches any digit (0 through 9).

• Example:

  • [aeiou] matches any vowel.

  • [A-Za-z] matches any letter (uppercase or lowercase).

2. Grouping ()

• Definition: Groups part of a regex for applying quantifiers or capturing matched text.

• Syntax: (abc) matches the exact sequence "abc".

• Alternation: (cat|dog) matches either "cat" or "dog".

• Example:

  • (abc)+ matches "abc" one or more times (e.g., abc, abcabc).

3. Quantifiers

• Definition: Specifies how many times the preceding element should be matched.

• Common Quantifiers:

  • * - Zero or more times.

  • + - One or more times.

  • ? - Zero or one time.

  • {n} - Exactly n times.

  • {n,} - At least n times.

  • {n,m} - Between n and m times.

• Example:

  • a* matches zero or more as.

  • a+ matches one or more as.

  • \d{3} matches exactly three digits (e.g., 123).

Combining Concepts

• Example:

  • ([A-Za-z]+)\s+([0-9]+) matches a word followed by one or more spaces and then digits.