Function is_alpha 

pub fn is_alpha(c: char) -> bool

Check whether the character is an ASCII alphanumeric, _ or -.

This is used for scanning tag handles and similar constructs. Note: This is slightly more permissive than YAML’s ns-word-char (which excludes _). For strict ns-word-char compliance, use [is_word_char] instead.

Matches: [0-9a-zA-Z_-]