lwcInputCustomValidity.js

import { LightningElement } from 'lwc';

export default class lwcInput extends LightningElement {
    handleChange(event) {
        let inputCompValue = event.target.value;
        let inputComp = this.template.querySelector('.inputComp');
        let splChars = /[0-9(@!#\$%\^\&*\)\(+=._-]/;
        if(inputCompValue.match(splChars)) {
            inputComp.setCustomValidity("Only character allowed.");
            inputComp.reportValidity();
        } else {
            inputComp.setCustomValidity('');
            inputComp.reportValidity(); 
        }
    }
}