正则表达式。替换在勉强匹配时有奇怪的行为

Question

Answering to this question I stuck with this situation. Using reluctant match in my regex bring to this result

string s = Regex.Replace(".A.", "\\w*?", "B");

B.BAB.B

Why it doesn't match and replace A?

Answer 1

Because the \\\\w*? matches as few \\w as it possibly can, including 0 of them .

Since you have \\w* instead of \\w+ , the regex matches 0 or more \\w .

Since you have an additional ? on the \\w* , the smallest possible match for this regex is the 0-length string, ''.

Since the ? forces the regex to match as small a match as possible, it only ever matches 0-length strings. It can't match a single character A because that would be a longer match than the shortest.

Hence all 0-length strings in .A. (being: ''.''A''.'' , where each possible 0-length string is marked as '' ) are replaced with a 'B', giving you 'BAB'.

If you want to disable this behaviour and replace at least one \\w , you can use regex \\w+? . However, by the same reasoning as before, the ? forces this to only ever replace \\w of length one, so you may as well use regex \\w .