Javascript Regex exec weird

Question

I don't understand the behaviour of Javascript Regex

Im validating a text with this code:

let validLine = /^-*[\w\s]+:(concept|term):-?(0[.]?\d*|1)$/g
let isValid = newText
  .split('\n')
  .reduce((valid, line) => {
    line = line.trim()
    if (line === '') return valid
    return valid && !!validLine.exec(line)
}, true)

But this code does not behave as intended. The following, where I initialize the regex everytime does

let isValid = newText
.split('\n')
.reduce((valid, line) => {
  let validLine = /^-*[\w\s]+:(concept|term):-?(0[.]?\d*|1)$/g
  line = line.trim()
  if (line === '') return valid
  return valid && !!validLine.exec(line)
}, true)

Why?