“(?...） 的正则表达式解释“和”(2）”

我需要一些帮助来理解一些正则表达式。我看到了这样的代码

preg_match("/^ 
            (1[-'s.])?  # optional '1-', '1.' or '1' 
            ( '( )?     # optional opening parenthesis 
            'd{3}       # the area code 
            (?(2) ') )  # if there was opening parenthesis, close it 
            [-'s.]?     # followed by '-' or '.' or space 
            'd{3}       # first 3 digits 
            [-'s.]?     # followed by '-' or '.' or space 
            'd{4}       # last 4 digits 
            $/x",$number)

我都明白了，但不明白(?(2) ') )是如何工作的......咦？（2）代表。

问题更新...

当我像这样更改代码时，我阅读了您的所有答案

preg_match("/^ 
            (1[-'s.])?  # optional '1-', '1.' or '1' 
            'd{3}       # the area code 
            ( '( )?     # optional opening parenthesis 
            (?(3) ') )  # if there was opening parenthesis, close it 
            [-'s.]?     # followed by '-' or '.' or space 
            'd{3}       # first 3 digits 
            [-'s.]?     # followed by '-' or '.' or space 
            'd{4}       # last 4 digits 
            $/x",$number)

我收到类似错误

Compilation failed: reference to non-existent subpattern   

代码有什么问题吗？