I want to prevent numbers and special characters in an input, to force a character in the first position

I have already been successful in doing these other things:

• prevented first character from being a space using javascript
• prevented consecutive spaces using javascript
• forced each word in the input to be capitalized using html
  Now I want to force the first character to be forced a letter, to prevent a number and special character only in the first character.
  I know it can be done but I can not find this helpful code. I have done so much and this is the last thing I need for my form! 😀

This is how I did all of the mentioned above:

function SpaceBlock() {
    var space = document.getElementById("real_name");
        if(space.selectionStart === 0 && window.event.code === "Space"){
            window.event.preventDefault(); } }

function SpaceBlock2() {
    var space = document.getElementById("display_name");
        if(space.selectionStart === 0 && window.event.code === "Space"){
            window.event.preventDefault();} }
            
var lastkey;
var ignoreChars = ' rn'+String.fromCharCode(0);
function ignoreSpaces(e) {
    e = e || window.event;
    var char = String.fromCharCode(e.charCode);
    if(ignoreChars.indexOf(char) >= 0 && ignoreChars.indexOf(lastkey) >= 0) {
        lastkey = char;
        return false; }
    else {
        lastkey = char;
        return true; } }

var lastkey2;
var ignoreChars2 = ' rn'+String.fromCharCode(0);
function ignoreSpaces2(e) {
    e = e || window.event;
    var char2 = String.fromCharCode(e.charCode);
    if(ignoreChars2.indexOf(char2) >= 0 && ignoreChars2.indexOf(lastkey2) >= 0) {
        lastkey2 = char2;
        return false; }
    else {
        lastkey2 = char2;
        return true; } }

<input type="text" name="real_name" placeholder="Real Name" id="real_name" required minlength="6" maxlength="24" tabindex="1" onkeydown="SpaceBlock()" onkeypress="return ignoreSpaces(event);" style="text-transform: capitalize;" >

<input type="text" name="display_name" placeholder="Display Name" id="display_name" required minlength="6" maxlength="24" tabindex="1" onkeydown="SpaceBlock()" onkeypress="return ignoreSpaces(event);" style="text-transform: capitalize;" >

This is the last thing I need for my form, just hoping someone will help. Thanks!