> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Bash Character Class
A **bracket expression** is a pattern matching construct enclosed in square brackets (`[...]`) that matches exactly one character from a specified set. Within a bracket expression, a **character class** specifically refers to a named, locale-aware set of characters enclosed in `[:` and `:]`.
## Literal Sets and Ranges
A standard bracket expression matches any single literal character listed within the brackets. Ranges define a contiguous sequence of characters using a hyphen (`-`), determined by the collating order of the current locale.
```bash theme={"dark"}
[abc]
[a-z]
[a-zA-Z0-9_]
```
## Complementation (Negation)
A bracket expression is negated by placing an exclamation mark (`!`) or a caret (`^`) immediately after the opening bracket. This matches any single character *not* specified in the set. In standard Bash globbing, `!` is the POSIX-compliant negation operator, while `^` is typically used in regular expression contexts.
```bash theme={"dark"}
[!abc]
[^0-9]
```
## Character Classes
A character class is a predefined set of characters denoted by `[:classname:]`. Because character classes are locale-dependent, they match characters based on the system's linguistic rules rather than strict ASCII values. In a UTF-8 environment, a class like `[:alpha:]` will match accented characters (e.g., `é` or `ñ`), whereas a literal range like `[a-zA-Z]` may not. A character class must be nested inside a bracket expression to function.
```bash theme={"dark"}
[[:digit:]]
[[:lower:][:upper:]]
[![:space:]]
```
**Standard POSIX Classes:**
* `[:alnum:]`: Alphanumeric characters.
* `[:alpha:]`: Alphabetic characters.
* `[:blank:]`: Space and tab characters.
* `[:cntrl:]`: Control characters.
* `[:digit:]`: Decimal digits.
* `[:graph:]`: Printable characters excluding space.
* `[:lower:]`: Lowercase alphabetic characters.
* `[:print:]`: Printable characters including space.
* `[:punct:]`: Punctuation characters.
* `[:space:]`: Whitespace characters (space, tab, newline, carriage return, form feed, vertical tab).
* `[:upper:]`: Uppercase alphabetic characters.
* `[:xdigit:]`: Hexadecimal digits.
**Bash Extensions:**
* `[:ascii:]`: ASCII characters (range `0-127`).
* `[:word:]`: Alphanumeric characters plus underscore.
## Equivalence Classes
An equivalence class is denoted by `[=c=]` and matches all characters that share the same base collation weight as the character `c` according to the current locale. This is used to group a base letter with its accented variants.
```bash theme={"dark"}
[[=e=]]
```
*(Matches `e`, `é`, `è`, `ê`, etc., depending on the locale.)*
## Collating Symbols
A collating symbol is denoted by `[.symbol.]` and is used to match multi-character collating elements as a single entity within a bracket expression. This supports languages where two characters are treated as a single sorting unit (e.g., `ch` or `ll` in traditional Spanish collation).
```bash theme={"dark"}
[[.ch.]]
```
## Special Character Placement Rules
To match characters that normally hold syntactic meaning within a bracket expression (`]`, `-`, `!`, `^`), they must be placed in specific positions to force literal interpretation:
* **Closing Bracket (`]`):** Must be the first character in the set (or immediately following the negation operator).
```bash theme={"dark"}
[][] # Matches ']' or '['
[!]] # Matches any character except ']'
```
* **Hyphen (`-`):** Must be the first or last character in the set to avoid being interpreted as a range operator.
```bash theme={"dark"}
[-abc] # Matches '-', 'a', 'b', or 'c'
[abc-] # Matches 'a', 'b', 'c', or '-'
```
* **Exclamation/Caret (`!`, `^`):** Must be placed anywhere *except* the first position to be treated as literal characters.
```bash theme={"dark"}
[a!b^] # Matches 'a', '!', 'b', or '^'
```
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