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# Bash Negated Character Class
A negated character class in Bash is a pattern matching construct that evaluates to exactly one character *not* present in a specified set or range. It is evaluated by the Bash pattern matching engine during pathname expansion (globbing), `case` statement execution, conditional expression matching (`[[ string == pattern ]]`), and parameter expansion (e.g., `${var#pattern}`, `${var%pattern}`, `${var//[!0-9]/}`).
```bash theme={"dark"}
[!CHARACTERS]
[^CHARACTERS]
```
## Technical Mechanics
* **Negation Operators:** The exclamation mark (`!`) is the POSIX-compliant negation operator for shell pattern matching. Bash also accepts the caret (`^`) to mirror regular expression syntax, but `!` is required for strict POSIX portability.
* **Positional Requirement:** The negation operator (`!` or `^`) must be the immediate first character following the opening bracket `[`. If placed anywhere else within the brackets, it loses its negation property and is parsed as a literal character.
* **Arity:** A negated character class always consumes exactly one character in the target string. It cannot match zero characters, nor can it match multiple characters.
## Syntax Variations
**Discrete Sets**
Matches any single character that is not explicitly listed.
```bash theme={"dark"}
[!abc] # Matches any character except 'a', 'b', or 'c'
```
**Character Ranges**
Matches any single character whose collation value falls outside the defined start and end boundaries. Bash evaluates character ranges using the current locale's collation sequence (determined by the `LC_COLLATE` environment variable). In many common locales (such as `en_US.UTF-8`), this utilizes dictionary sorting (e.g., `aAbBcC`), meaning a negated range like `[!A-Z]` may unexpectedly exclude lowercase letters. Strict ASCII/Unicode byte-order evaluation is only applied if the locale is explicitly set to `C` or `POSIX`.
```bash theme={"dark"}
[!0-9] # Matches any character outside the range '0' through '9'
```
**Combined Sets and Ranges**
Multiple ranges and discrete characters can be combined within a single negated class.
```bash theme={"dark"}
[!a-fA-F0-9_] # Matches any character that is not a hex digit or underscore
```
## Parsing Rules for Special Characters
When constructing a negated character class, certain characters require specific positioning to be treated as literals rather than syntax operators:
* **Literal Hyphen (`-`):** To exclude a literal hyphen without creating an invalid range, the hyphen must be placed immediately after the negation operator or at the very end of the class.
```bash theme={"dark"}
[!-abc] # Excludes '-', 'a', 'b', and 'c'
[!abc-] # Identical parsing to the above
```
* **Literal Closing Bracket (`]`):** To exclude a literal closing bracket, it must be placed immediately after the negation operator. Bash parses the first `]` it encounters as the end of the class unless it is in this specific position.
```bash theme={"dark"}
[!]] # Excludes the literal character ']'
```
* **Literal Exclamation/Caret (`!` / `^`):** If these characters appear anywhere other than the first position, the parser treats them as standard literal characters to be included in the set.
```bash theme={"dark"}
[a!b] # NOT NEGATED. Matches exactly 'a', '!', or 'b'
[!a!b] # NEGATED. Excludes 'a', '!', and 'b'
```
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