‘\cC’ matches any character whose category is C. Here C is a character that represents a category: thus, ‘c’ for Chinese characters or ‘g’ for Greek characters in the standard category table. You can see the list of all the currently defined categories with ‘M-x describe-categories <RET>’. You can also define your own categories in addition to the standard ones using the ‘define-category’ function (*note Categories::). ‘[ ... ]’ is a “character alternative”, which begins with ‘[’ and is terminated by ‘]’. In the simplest case, the characters between the two brackets are what this character alternative can match. Thus, ‘[ad]’ matches either one ‘a’ or one ‘d’, and ‘[ad]*’ matches any string composed of just ‘a’s and ‘d’s (including the empty string). It follows that ‘c[ad]*r’ matches ‘cr’, ‘car’, ‘cdr’, ‘caddaar’, etc.
Example: ht 删掉 t 我 p 删掉 s 我: 删我 / 删我 /w 删我 w 删我 w 删我. 删我 ex 删我 am 删我 ple 删我 . 删我 co 删掉 m 我 / 删我 i7 删我 mx 删我 ss 删我 d M-x isearch-forward-regexp RET \(\cc\| \) then replace it with empty string gives the result of `https://www.example.com/i7mxssd`
but I wonder how to use
 instead of