JSFiddle - React, Tailwind, and code Playground

JavaScript

function simpleSymbols(str) {
    for (i = 0; i < str.length; i++) {
        if (isLetter(str[i]) && (str[i - 1] !== "+"  || str[i + 1] !== "+")) {
            return false;
        } 
    }
    return true;
}
function isLetter(str) {
  return str.length === 1 && str.match(/[a-z]/i);
}
console.log(simpleSymbols("+a+b+a+"));