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The following character class names are always supported by Boost.Regex:
| Name | POSIX-standard name | Description | 
|---|---|---|
| alnum | Yes | Any alpha-numeric character. | 
| alpha | Yes | Any alphabetic character. | 
| blank | Yes | Any whitespace character that is not a line separator. | 
| cntrl | Yes | Any control character. | 
| d | No | Any decimal digit | 
| digit | Yes | Any decimal digit. | 
| graph | Yes | Any graphical character. | 
| l | No | Any lower case character. | 
| lower | Yes | Any lower case character. | 
|  | Yes | Any printable character. | 
| punct | Yes | Any punctuation character. | 
| s | No | Any whitespace character. | 
| space | Yes | Any whitespace character. | 
| unicode | No | Any extended character whose code point is above 255 in value. | 
| u | No | Any upper case character. | 
| upper | Yes | Any upper case character. | 
| w | No | Any word character (alphanumeric characters plus the underscore). | 
| word | No | Any word character (alphanumeric characters plus the underscore). | 
| xdigit | Yes | Any hexadecimal digit character. |