# Identifiers

> **<sup>Lexer:<sup>**  
> IDENTIFIER_OR_KEYWORD :  
> &nbsp;&nbsp; &nbsp;&nbsp; [`a`-`z` `A`-`Z`]&nbsp;[`a`-`z` `A`-`Z` `0`-`9` `_`]<sup>\*</sup>  
> &nbsp;&nbsp; | `_` [`a`-`z` `A`-`Z` `0`-`9` `_`]<sup>+</sup>  
>  
> IDENTIFIER :  
> IDENTIFIER_OR_KEYWORD <sub>*Except a [strict] or [reserved] keyword*</sub>

An identifier is any nonempty ASCII string of the following form:

Either

* The first character is a letter.
* The remaining characters are alphanumeric or `_`.

Or

* The first character is `_`.
* The identifier is more than one character. `_` alone is not an identifier.
* The remaining characters are alphanumeric or `_`.

[strict]: keywords.html#strict-keywords
[reserved]: keywords.html#reserved-keywords
