Max length ignoring one character in Regex - regex

Question

I have a Regex Which allows up to 6 decimals("." is the decimal separator)
/^\d*[.]?\d{0,6}$/
I also want to put max length condition so that user can only enter 12 digits and max length should exclude "." how do I do that with Regex.

Answer 1

You could use a positive lookahead to check for either a number with a decimal place with up to 6 digits after it or a string of 12 digits) and then match up to 13 characters in total:
^(?=\d*\.\d{0,6}$|\d{1,12}$).{1,13}$
For this input, the 2nd and 5th values will match:
1234567890123
123456.789012
12345.6789012
1234567.890123
12345.67890
Demo on regex101

Answer 2

We could try using negative lookaheads:
^(?:(?!.*\.)(?!\d{13})|(?=.*\.)(?![0-9.]{14}))\d+(?:\.\d{1,6})?$
Demo
Here is an explanation of the regex:
^(?: from the start of the string
(?!.*\.)(?!\d{13}) assert that no more than 12 digits appear
(in the case of a number with NO decimal point)
| or
(?=.*\.)(?![0-9.]{14})) assert that no more than 13 digits/decimal point appears
(in the case of a number which HAS a decimal point)
\d+ then match one or more digits (whole number portion)
(?:\.\d{1,6})? followed by an optional decimal component (1 to 6 digits)
$ end of the string

Answer 3

TL;DR;
^(?!(?:\D*\d){13})\d*[.]?\d{0,6}$
^(?=(?:\D*\d){0,12}\D*$)\d*[.]?\d{0,6}$
You can use a simple positive lookahead approach: keep your pattern (if it works as you expect) and insert
(?=(?:\D*\d){0,x}\D*$)
right after ^ and change the x to the required amount of digits.
So, you may use
^(?=(?:\D*\d){0,12}\D*$)\d*[.]?\d{0,6}$
^^^^^^^^^^^^^^^^^^^^^^^
The (?=(?:\D*\d){0,12}\D*$) matches a location that is immediately followed with zero to 12 occurrences of any 0+ non-digit chars followed with one digit one and then having any 0+ non-digits to the end of the string.
See the regex demo
Alternatively, disallow a string with more than 13 digits:
^(?!(?:\D*\d){13})\d*[.]?\d{0,6}$
^^^^^^^^^^^^^^^^^
The (?!(?:\D*\d){13}) is a negative lookahead that fails the match if there are 13 occurrences of any 0+ non-digits followed with a single digit chars.
This is better than the positive lookahead approach when you need to allow an empty string.
See the regex demo