Partial matching a string against a regex

Looks like you're lucky, I've already implemented that stuff in JS (which works for most patterns - maybe that'll be enough for you). See my answer here. You'll also find a working demo there.

There's no need to duplicate the full code here, I'll just state the overall process:

• Parse the input regex, and perform some replacements. There's no need for error handling as you can't have an invalid pattern in a RegExp object in JS.
• Replace abc with (?:a|$)(?:b|$)(?:c|$)
• Do the same for any "atoms". For instance, a character group [a-c] would become (?:[a-c]|$)
• Keep anchors as-is
• Keep negative lookaheads as-is

Had JavaScript have more advanced regex features, this transformation may not have been possible. But with its limited feature set, it can handle most input regexes. It will yield incorrect results on regex with backreferences though if your input string ends in the middle of a backreference match (like matching ^(\w+)\s+\1$ against hello hel).

I think that you have to have 2 regex one for typing /a?b?c?d?/ and one for testing at end while paste or leaving input /abcd/

This will test for valid phone number:

const input = document.getElementById('input')

let oldVal = ''
input.addEventListener('keyup', e => {
  if (/^\d{0,3}-?\d{0,3}-?\d{0,3}$/.test(e.target.value)){
    oldVal = e.target.value
  } else {
    e.target.value = oldVal
  }
})
input.addEventListener('blur', e => {
  console.log(/^\d{3}-?\d{3}-?\d{3}-?$/.test(e.target.value) ? 'valid' : 'not valid')
})

<input id="input">

And this is case for name surname

const input = document.getElementById('input')

let oldVal = ''
input.addEventListener('keyup', e => {
  if (/^[A-Z]?[a-z]*\s*[A-Z]?[a-z]*$/.test(e.target.value)){
    oldVal = e.target.value
  } else {
    e.target.value = oldVal
  }
})
input.addEventListener('blur', e => {
  console.log(/^[A-Z][a-z]+\s+[A-Z][a-z]+$/.test(e.target.value) ? 'valid' : 'not valid')
})

<input id="input">

As many have stated there is no standard library, fortunately I have written a Javascript implementation that does exactly what you require. With some minor limitation it works for regular expressions supported by Javascript. see: incr-regex-package.

Further there is also a react component that uses this capability to provide some useful capabilities:

1. Check input as you type
2. Auto complete where possible
3. Make suggestions for possible input values

Demo of the capabilities Demo of use