Regular expression to accept only characters (a-z) in a textbox

The pattern itself would be [a-z] for single character and ^[a-z]+$ for entire line. If you want to allow uppercase as well, make it [a-zA-Z] or ^[a-zA-Z]+$

Try this to allow both lower and uppercase letters in A-Z:

/^[a-zA-Z]+$/

Remember that not all countries use only the letters A-Z in their alphabet. Whether that is an issue or not for you depends on your needs. You may also want to consider if you wish to allow whitespace (\s).