正则表达式“（？<！^）（？= [A-Z]）”

Question

I spent like three hours trying to understant how does "(?<!^)(?=[AZ])" works to split at tring according to capital letters ie

string[] s = Regex.Split("TheWorldWithoutStrangers", "(?<!^)(?=[A-Z])");

How does it work !! I do understand what is the meaning of each char in the above expression, but I do not get how does it work together. why "(? < !^)([AZ])" doesnot work ? it means that whenever you find a captial letter that is not after a new line, then split, am I right ?

Answer 1

The ^ means the beginning of a line, and (?<!...) is a negative look behind, so (?<!^) matches any position in the string that is not right at the beginning of the line.

The set [AZ] matchies any capital letter, and (?=...) is a positive look ahead, so (?=[AZ]) matches any position in the string that is right before a capital letter.

Put them together, and the expression matches any position in the string that is not right at the beginning of a line, and that is right before a capital letter.

Answer 2

The key here is that the two parts (?<!...) and (?=...) are zero-width assertions. The first one makes sure ^ (start of string) does not occur right before the match position and the second one ensures that [AZ] (single capital letter) appears right after the match position. The actual match is empty because neither of the assertions actually match any characters. The entire expression merely matches a position.