How to make the right mask?

Question

I'm checking valid strings for "Invalid format".

The minimum length of the string is 5 characters, the maximum is 32 (/[a-z\d_\.]{5,32}/).

The "Invalid format" error occurs. It occurs if the string:

1. Starts with three or more digits (one or two digits are acceptable);
2. It starts and ends with the _ character;
3. Contains a dot after which there are less than four characters starting with the letter.

I wrote an expression that works for the first two conditions, but I don't understand how to implement the third condition? If possible, then give an example of a separate expression, I do not understand exactly how to do it with the dot symbol.

^(?!(?:_|\d{3,}))[a-z\d_\.]{5,32}(?<!_)$

https://regex101.com/r/W9njMe/1

Thanks.

Answer 1

You might use a pattern with an assertion, that does not match a dot after the last occurence of [a-z] when there are 1-4 chars left till the end of the string.

^(?!_|\d{3})(?!.*[a-z](?=.{1,4}$)[a-z\d_]*\.)[a-z\d_.]{5,32}$(?<!_)

Explanation

• ^ Start of string
• (?!_|\d{3}) Assert not _ or 3 digits
• (?! Negative lookahead, assert what is to the right is not
  • .*[a-z] Match the last occurrence of [a-z]
  • (?=.{1,4}$) Assert 1-4 chars till end of string
  • [a-z\d_]*\. Match match a .
• ) Close lookahead
• [a-z\d_.]{5,32}$ Match 5-32 chars in the string
• (?<!_) Assert not _ to the left

Regex demo