Regular expression: Match a string that contains no more than three consecutive Bs (without using negative lookahead)

Question

This is a simple question but I just recently started learning about regex.

I need to give a regex that describes all strings that contain no more than three consecutive Bs.

It should match:

sdBBdfsBBB

BfsBBBdsfBBB

It shouldn't match:

fsBBBBsfBB

BBBBBfsBBBsd

I am not allowed to use negative lookahead, only fundamental operations and shortcuts and I have no idea how to solve this... I know that the regex .*B{1,3}.* would match the strings that I want my regex to match but it would also match those I do not want my regex to match...

Answer 1

To not match empty strings, you could match either repeated sets of 1-3 times a B with at least any char except a B in between or match a string that ends with 1-3 times a B

^(?:[^B]*(?:B{1,3}[^B]+)+B{0,3}|[^B]*B{1,3})$

• ^ Start of string
• (?: Non capture group
  • [^B]* Match 0+ times any char except B
  • (?: Non capture group
    • B{1,3}[^B]+ Repeat 1+ times 1-3 B's and 1+ times any char except B
  • )+ Close group and repeat 1+ times
  • B{0,3} Match 0-3 B's at the end
  • | Or
  • [^B]*B{1,3} Match 0+ times any char except B and then 1-3 times a B
• ) Close group
• $ End of string

Regex demo

If empty string are allowed, the pattern could be shortened to

^(?:[^B]*(?:B{1,3}[^B]+)*B{0,3})$

Regex demo

Note that negated character class [^B] matches any char except a B and could possibly also match a newline. To not allow newlines to match, use [^B\r\n] or [^B\s] to exclude whitespace chars.