Question

I can’t understand how to solve the following problem:

I have input string “aaaabaa” and I’m trying to search for string “aa” (I’m looking for positions of characters)
Expected result is
0 1 2 5

• aa aabaa
• a aa abaa
• aa aa baa
• aaaab aa

This problem is already solved by me using another approach (non-RegEx).
But I need a RegEx I’m new to RegEx so google-search can’t help me really.
Any help appreciated! Thanks!

P.S.
I’ve tried to use (aa)* and "\b(\w+(aa))*\w+" but those expressions are wrong

Answer 1

You can solve this by using a lookahead

a(?=a)

will find every “a” that is followed by another “a”.

If you want to do this more generally

(\p{L})(?=\1)

This will find every character that is followed by the same character. Every found letter is stored in a capturing group (because of the brackets around), this capturing group is then reused by the positive lookahead assertion (the (?=...)) by using \1 (in \1 there is the matches character stored)

\p{L} is a unicode code point with the category “letter”

Code

String text = "aaaabaa";
Regex reg = new Regex(@"(\p{L})(?=\1)");

MatchCollection result = reg.Matches(text);

foreach (Match item in result) {
    Console.WriteLine(item.Index);
}

Output

0
1
2
5