JSFiddle - React, Tailwind, and code Playground

JavaScript

function testa(str) {
    var match = str.match(/(\d)/g);
    return !!match && match.length == 1;
}

var a = "1"; 
var b = "sp 1 inf";
var c = "sp 1 inf 5";
var d = "sp inf";

console.log(testa(a)); // true
console.log(testa(b)); // true
console.log(testa(c)); // false
console.log(testa(d)); // false