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# 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 `a`s.
  * `a+` matches one or more `a`s.
  * `\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.