RegEx for decimal number with maximum length ignoring dot

Question

I am after a regex that allows a number to be a maximum length with decimal places.

It should allow 16 numbers maximum and 3 decimal places, however the 16 max should include the decimals but not the . character. I've tried and have this so far: ^(?=^[\d\.].{0,16}$)[0-9]+(\.[0-9]{1,3})?$, which is close except it allows 17 single numbers, when it should only accept 16. If I change {0,16} to {0,15} it then breaks the decimals.

Accepted values:

• 123
• 123.3
• 123.45
• 123.456
• 1111111111111.345 (totally 16 numbers)
• 11111111111111.45 (totally 16 numbers)

Rejected:

• 123.1234
• 11111111111111111 (17 characters)
• 1111111111111111.
• 111111111111111.56 (15 characters and 2 numbers = 17)

Thanks in advance.

Answer 1

Your regex is fairly close, you may use this regex:

^(?=(?:\d\.?){0,16}$)\d+(?:\.\d{1,3})?$

RegEx Demo

RegEx Details:

• (?=(?:\d\.?){0,16}$): is positive lookahead to assert that we have 0 to 16 digits in input where dot is optional.
• \d+(?:\.\d{1,3})?: Match an integer or a decimal number with 1 to 3 digits.