Check if a value is within a range of numbers

You’re asking a question about numeric comparisons, so regular expressions really have nothing to do with the issue. You don’t need “multiple if” statements to do it, either:

if (x >= 0.001 && x <= 0.009) {
  // something
}

You could write yourself a “between()” function:

function between(x, min, max) {
  return x >= min && x <= max;
}
// ...
if (between(x, 0.001, 0.009)) {
  // something
}