It could be done in efficient time complexity using KMP algorithm in O(N + M) where N is the length of text and M is the length of the pattern.
This is the implementation and usage:
static class StringExtensions
{
public static IEnumerable<int> AllIndicesOf(this string text, string pattern)
{
if (string.IsNullOrEmpty(pattern))
{
throw new ArgumentNullException(nameof(pattern));
}
return Kmp(text, pattern);
}
private static IEnumerable<int> Kmp(string text, string pattern)
{
int M = pattern.Length;
int N = text.Length;
int[] lps = LongestPrefixSuffix(pattern);
int i = 0, j = 0;
while (i < N)
{
if (pattern[j] == text[i])
{
j++;
i++;
}
if (j == M)
{
yield return i - j;
j = lps[j - 1];
}
else if (i < N && pattern[j] != text[i])
{
if (j != 0)
{
j = lps[j - 1];
}
else
{
i++;
}
}
}
}
private static int[] LongestPrefixSuffix(string pattern)
{
int[] lps = new int[pattern.Length];
int length = 0;
int i = 1;
while (i < pattern.Length)
{
if (pattern[i] == pattern[length])
{
length++;
lps[i] = length;
i++;
}
else
{
if (length != 0)
{
length = lps[length - 1];
}
else
{
lps[i] = length;
i++;
}
}
}
return lps;
}
and this is an example of how to use it:
static void Main(string[] args)
{
string text = "this is a test";
string pattern = "is";
foreach (var index in text.AllIndicesOf(pattern))
{
Console.WriteLine(index); // 2 5
}
}