JSFiddle - React, Tailwind, and code Playground
HTML
<input type="text" name="toto" id="titi" >
<input type="button" onclick="VerifStr()">
JavaScript
function VerifStr() {
var valeur = document.getElementById('titi').value;
var exp=new RegExp("^[a-zA-Z]*$","g");
if ( exp.test(valeur) ) {
alert("OK");
}else {
alert("Erreur");
}
}