Javascript Regular expression for basic digits with decimal points?

Question

I am trying to use a regular expression to validate decimal values . I wrote below regular expression but it does not allowing first decimal with value like a .5 or .6 or .1

Regular Exp : /^\d[0-9]{0,13}(\.\d{1,2})?$/

Rules :

1. It should allow positive numbers.
2. It should allow max 13 numbers before decimal point
3. It should allow max two number after decimal.
4. It should allow .(dot) with number like a .5
5. It should not allow the .0

Example - Valid inputs

• 0
• 0.5
• 1.55
• .5
• 1234567890123(13 numbers before decimal)
• 1234567890123.5
• 1234567890123.00

Example - invalid inputs

• .(dot),
• .0
• 1.234
• 5.
• 12345678901234(14 numbers before decimal)
• 12345678901234.56

const valid = [
  "0",
  "0.5",
  "1.55",
  ".5",
  "1234567890123",
  "1234567890123.5",
  "1234567890123.00",
];

const invalid = [
  ".",
  ".0",
  "1.234",
  "5.",
  "12345678901234",
  "12345678901234.56",
];

const rgx = /^\d[0-9]{0,13}(\.\d{1,2})?$/

console.log("Checking valid strings (should all be true):");
valid.forEach(str => console.log(rgx.test(str), str));

console.log("\nChecking invalid strings (should all be false):");
invalid.forEach(str => console.log(rgx.test(str), str));

Answer 1

I believe the following regex should meet all of your criteria:

^(\d{1,13}($|\.\d?\d$)|\.[1-9]\d?$)

first case: 1-13 digits followed either by nothing or by a "." followed by one or two digits

second case: a "." followed by a non zero digit and at most one other digit

const valid = [
  "0",
  "0.5",
  "1.55",
  ".5",
  "1234567890123",
  "1234567890123.5",
  "1234567890123.00",
];

const invalid = [
  ".",
  ".0",
  "1.234",
  "5.",
  "12345678901234",
  "12345678901234.56",
];

const rgx = /^(\d{1,13}($|\.\d?\d$)|\.[1-9]\d?$)/

console.log("Checking valid strings (should all be true):");
valid.forEach(str => console.log(rgx.test(str), str));

console.log("\nChecking invalid strings (should all be false):");
invalid.forEach(str => console.log(rgx.test(str), str));

Answer 2

i think this should do most of your requirment but not all of them, limit to 9 decimal places

( /^(\d+\.?\d{0,9}|\.\d{1,9})$/ )

and this one with no decimal limit

( /^(\d+\.?\d*|\.\d+)$/ )