正则表达式：一个字符在一个序列中不能出现超过 4 次

Question

I am creating a regular expression where the string MUST start with E, can have unlimited E's (E+), and MUST end with C ([C]).

Then B can OPTIONALLY be between E and C, but only in pairs (B{2}?).

Now this is the part I'm stuck on. There cannot be more than 4 P's in the string. I am trying to use S{0,4}.

Lastly, each P or sequence of P's in the series MUST be followed by a D.

I've been experimenting a lot but can't figure out the P and D.

Answer 1

You can use

^E+(?!(?:[^P\n]*P){5})(?:B{2}|P+D)*C$

• (??(::[^P\n]*P){5}) - Make sure there are no more than 5 Ps on this line. Do this in a negative lookahead , rather than consuming the characters, to allow for more logic

• (?:B{2}|P+D)* - Every substring in the middle either needs to be a B pair, or some Ps followed by a D

https://regex101.com/r/2tg7Mr/1