Replace when wild cards used

In excel, ? matches any single character, thus s?b could be used to find s1b, s2b, and so forth. Once the matching items are found, I would like to leave the value matched by the ? wild card unchanged, let's say, for example, I want s1b to be replaced by s1foo, s2b by s2foo, and so forth. I tried using s?foo as the replacement string, but that literally replaced the digit with the ? character rather than leaving it alone. Is there a character that indicates "no change" to what matched the wildcard?