You could use LINQ to be nice and expressive.
In order to use this make sure you have a using System.Linq at the top of your code.
Assuming that
source is the stored target patterntest is the string to test.
Then you can do
public static bool IsValid(string source, string test)
{
return test != null
&& source != null
&& test.Length == source.Length
&& test.Where((x,i) => source[i] != x).Count() <=2
}
There is also a shortcut version that exits false the moment it fails, saving iterating the rest of the string.
public static bool IsValid(string source, string test)
{
return test != null
&& source != null
&& test.Length == source.Length
&& !test.Where((x,i) => source[i] != x).Skip(2).Any();
}
As requested in comments, a little explanation of how this works
in C# a string can be treated as an array of characters, which means that the Linq methods can be used on it.
test.Where((x,i) => source[i] != x)
This uses the overload of Where that for each character in test, x gets assigned to the character and i gets assigned to the index. If the condition character at position i in source is not equal to x then output into the result.
Skip(2)
this skips the first 2 results.
Any()
this returns true if there any results left or false if not. Because linq defers execution the moment that this is false the function exits rather than evaluating the rest of the string.
The entire test is then negated by prefixing with a '!' to indicate we want to know where there are no more results.
Now in order to match as substring you are going to need to behave similar to a regex backtracking...
public static IEnumerable<int> GetMatches(string source, string test)
{
return from i in Enumerable.Range(0,test.Length - source.Length)
where IsValid(source, !test.Skip(i).Take(source.Length))
select i;
}
public static bool IsValid(string source, IEnumerable<char> test)
{
return test.Where((x,i) => source[i] != x).Skip(2).Any();
}
UPDATE Explained
Enumerable.Range(0,test.Length - source.Length)
This creates a sequence of numbers from 0 to test.Length - source.Length, there is no need in checking starting at every char in test because once the length is shorter the answer is invalid.
from i in ....
Basically iterate over the collection assigning i to be the current value each time
where IsValid(source, !test.Skip(i).Take(source.Length))
Filter the results to only include the ones where there is a match in test starting at index i (hence the skip) and going on for source.Length chars (hence the take.
select i
return i
This returns an enumerable over the indexes in test where there is a match, you could extract them with
GetMatches(source,test).Select(i =>
new string(test.Skip(i).Take(source.Length).ToArray()));
