I would try this onChange
function:
onChange={(e) => {
let value = e.target.value
value = value.replace(/[^A-Za-z]/ig, '')
this.setState({
value,
})
}}
See the codepen: https://codepen.io/bozdoz/pen/vzJgQB
The idea is to reverse your regex matcher with ^
and replace all non-A-z characters with ''