Getting overlapping regex matches in C#

Question

I have the regex 1(0*)1 and the test string 1000010001

I want to have 2 matches, but I find that only 1 gets found :

var regex = new Regex("1(0*)1");
var values = regex.Matches(intBinaryString);
// values only has 1 match

regexonline seems to agree : https://regex101.com/r/3J9Qxj/1

What am I doing wrong?

Answer 1

You are already selecting the 1 in front of the second zero by the first match.

100001 0001
^^^^^^

This is the first match. The rest is just 0001 which does not match your regex.

---

You can circumvent this behavior if you are using lookaheads/lookbehinds:

(?<=1)(0*)(?=1)

Live example

---

Because you cannot use lookbehinds in JavaScript, it is enough to only use one lookahead, to prevent the overlapping:

1(0*)(?=1)

Live example

---

And a hint for your regex101 example: You did not add the global flag, which prevents more than one selection.

Answer 2

You need to match overlapping strings.

It means you should wrap your pattern with a capturing group (( + your pattern + )) and put this consuming pattern into a positive lookahead, then match all occurrences and grab Group 1 value:

(?=(YOUR_REGEX_HERE))

Use

var regex = new Regex("(?=(10*1))");
var values = regex.Matches(intBinaryString)
    .Cast<Match>()
    .Select(m => m.Groups[1].Value)
    .ToList();

See the regex demo