I'd want to validate a username using a regular expression, however the expression I used worked well on https://regexr.com/ but not in my code. I just want user can add only Alphanumeric and a space between them
var letters = /^[A-Za-z]+$/g;
function validate() {
var input = document.getElementById("fname").value;
if (input.match(letters)) {
document.getElementById("name").innerHTML = "** Only alphabets are allowed in name.";
return false;
}
}<input id="fname">
<input type="button" onclick="validate()" value="Check">
<br>
<span id="name"></span>Your current function is "finding alphanumeric characters". So, when you run validate(), it is returning either a valid Array, or null. To get spaces use /^[\w\-\s]+$/ although this will return true for multiple spaces. I'm not sure your use case.
Unless you want to change your regex, you need to show your warning when input.match() returns a falsey value. To do this, Place ! before your input.match() function
var letters = /^[\w\-\s]+$/;
function validate() {
document.getElementById("name").innerHTML = "";
var input = document.getElementById("fname").value;
if (!input.match(letters)) {
document.getElementById("name").innerHTML = "** Only alphabets are allowed in name.";
return false;
}
}<input id="fname">
<input type="button" onclick="validate()" value="Check">
<br>
<span id="name"></span>