https://eloquentjavascript.net/09_regexp.html#i_izldJoT3uv

Eloquent JavaScript 0903 Numbers again.js

// Fill in this regular expression.
let number = /^[+-]?(\d+\.?\d*|\.\d+)(E[+-]?\d+)?$/i;
console.log(/\d/.test("."));

// Tests:
for (let str of ["1", "-1", "+15", "1.55", ".5", "5.",
                 "1.3e2", "1E-4", "1e+12"]) {
  if (!number.test(str)) {
    console.log(`Failed to match '${str}'`);
  }
}
for (let str of ["1a", "+-1", "1.2.3", "1+1", "1e4.5",
                 ".5.", "1f5", "."]) {
  if (number.test(str)) {
    console.log(`Incorrectly accepted '${str}'`);
  }
}