regex to start and end with character set, but with different character set in-between

Question

I'm trying to create a Regex that:

• must have it's first character in a set of characters (FIRST_SET)
• can optionally have follow-on characters in a different set of characters (SECOND_SET)
• but if it has 2 or more characters, the last character must be in the FIRST_SET
• can be no longer than MAX_CHARS characters in total

Example

• FIRST_SET = a-c or e-g (so d is excluded)
• SECOND_SET = a-g
• MAX_CHARS = 10

Here's what I have so far:

^[a-c|e-g][a-g]{0,8}[a-c|e-g]{0,1}$

This seems to work, EXCEPT if d is the last character and total character count < MAX_CHARS

Is there a way to fix this up?

Answer 1

You may use

^(?!.{11})(?=.*[a-ce-g]$)[a-ce-g][a-g]{0,9}$

See the regex demo.

Details

• ^ - start of string
• (?!.{11}) - up to 10 chars allowed
• (?=.*[a-ce-g]$) - after 0 or more chars, the last one should be from FIRST SET
• [a-ce-g] - a letter from the FIRST SET
• [a-g]{0,9} - zero to nine chars in the SECOND SET
• $ - end of string.

Note that | inside character classes match literal pipe chars, you need to remove it from your pattern.

The (?!.{11}) negative lookahead is executed once at the start of the string and fails the match if there are any 11 chars (other than newline) in the string. You may also use (?=.{0,10}$), it will require 0 to 10 chars in the string only.