The use of backslash are called shorthand) and specifies:
The syntax of a shorthand in a regular pattern.
\[a-zA-Z]??
where:
| Generic character types | Description | Equivalent to |
|---|---|---|
| \d | any decimal digit | [0-9] |
| \D | any character that is not a decimal digit | [^0-9] |
| \h | any horizontal whitespace character | |
| \H | any character that is not a horizontal whitespace character | |
| \s | any whitespace character | [\f\n\r\t\v\u00a0\u1680\u180e\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff] |
| \S | any character that is not a whitespace character | [^ \f\n\r\t\v\u00a0\u1680\u180e\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff] |
| \v | any vertical whitespace character | |
| \V | any character that is not a vertical whitespace character | |
| \w | any “word” character | [A-Za-z0-9_] |
| \W | any “non-word” character | [^A-Za-z0-9_] |
These character type sequences can appear both inside and outside character classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all of them fail, since there is no character to match.