With /^[a-zA-Z]/ you only check the first character:
^: Assert position at the beginning of the string[a-zA-Z]: Match a single character present in the list below:a-z: A character in the range between “a” and “z”A-Z: A character in the range between “A” and “Z”
If you want to check if all characters are letters, use this instead:
/^[a-zA-Z]+$/.test(str);
^: Assert position at the beginning of the string[a-zA-Z]: Match a single character present in the list below:+: Between one and unlimited times, as many as possible, giving back as needed (greedy)a-z: A character in the range between “a” and “z”A-Z: A character in the range between “A” and “Z”
$: Assert position at the end of the string (or before the line break at the end of the string, if any)
Or, using the case-insensitive flag i, you could simplify it to
/^[a-z]+$/i.test(str);
Or, since you only want to test, and not match, you could check for the opposite, and negate it:
!/[^a-z]/i.test(str);