what is the wildcard character that can be used in the filter to represent unknown length of characters?

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Are you using the matches operator in a display filter? (WSUG - Comparing Values)
It supports PCRE regular expressions. regex101 or CyberChef can be used to test the regex.

. - Matches any single character

.* - Matches Zero or more of any character

.+ - Matches one or more of any character

http.host matches "web.*net" - match strings containing web thennet with zero or more characters between