Using RegExp to get just one "-" at the start if needed

I want a number, a minimum of -1, and only a negative being inputed at the start.

This is what I have so far. The -1 minimum does not work. The single decimal point works great. The single negative sign works great. BUT I cannot force the negative to only appear at the start.

How can I get the negative only at the start if required? How can I get the minimum of -1 working?

<input type="text" id="mynumber" min="-1"
        oninput="this.value = this.value.replace(/[^0-9.-]/g, '').replace(/(\..*)\./g, '$1').replace(/(\--*)\-/g, '$1');" /> 

You could use 1 pattern for the first 2 calls to replace:

[^0-9.\n-]+|^-(?:1\.0*[1-9]|[2-9]|1\d+).*|(?!^)-

The pattern matches:

• [^0-9.\n-] Match 1+ chars not being any listed in the character class
• | Or
• ^-0* Match - at the start of the string followed by optional zeroes
• (?:1\.0*[1-9]|[2-9]|1\d+) Match either 1. followed by optional zeroes and a digit 1-9, or match a digit 2-9 or match a number greater than 10
• .* Match the rest of the line
• | Or
• (?!^)- Match a - not at the start of the string

See a regex demo.

Keep the last call to remove the second occurrence of the dot.

function handleInput() {
  this.value = this.value.replace(/[^0-9.\n-]+|^-0*(?:1\.0*[1-9]|[2-9]|1\d+).*|(?!^)-/g, '').replace(/(\..*)\./g, '$1');
}

const inp = document.getElementById("mynumber");

if (inp) {
  inp.addEventListener("input", handleInput);
}

<input type="text" id="mynumber" />

If a lookbehind assertion is supported in your JavaScript environment, you can use a single replace:

function handleInput() {
  this.value = this.value.replace(/[^0-9.\n-]+|^-0*(?:1\.0*[1-9]|[2-9]|1\d+).*|(?!^)-|(?<!^[^\n.]*)\./g, '');
}

const inp = document.getElementById("mynumber");

if (inp) {
  inp.addEventListener("input", handleInput);
}

<input type="text" id="mynumber" />