How to allow user key in first character alphabet/numeric and the rest must numeric?

Question

Hi i have a field that allows only alphabet or numeric in First character, and the rest only allow numeric only, and no allow symbols/space etc.

how to make the jquery/code?

Answer 1

Very simple regex:

/^[a-z0-9]\d*$/

Answer 2

Elaborating Jack answer little bit and convert to working snippet.

$('#yourid').on("keyup", function() {
  let val = $(this).val();
  let reg = /^[a-z0-9]\d*$/
  let newval = val.replace(reg, '');
  if (!val.match(reg)) {
    $(this).val(''); // clear the field if pattern don't match. 
  }

});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input type='text' id='yourid' />

How above regex work.

^ asserts position at start of the string

Match a single character present in the list below [a-z0-9]

a-z a single character in the range between a and z (case sensitive)

0-9 a single character in the range between 0 and 9 (case sensitive)

\d* matches a digit (equal to [0-9])

* Quantifier — Matches between zero and unlimited times, as many times as possible, giving back as needed

$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)