Question

Is there a theorical expression size limit for “or” operator on Regex.Replace
such as Regex.Replace(“abc”,”(a|c|d|e…continue say 500000 elements here)”,”zzz”) ?

Any stackoverflowException on .NET’s implementation ?

Thanks

Answer 1

There is no theoretical limit, though each regular expression engine will have its own implementation limits. In this case, since you are using .NET the limit is due to the amount of memory the .NET runtime can use.

A regular expression with one million alernations works fine for me:

string input = "a<142>c";
var options = Enumerable.Range(0, 1000000).Select(x => "<" + x + ">");
string pattern = string.Join("|", options);
string result = Regex.Replace(input, pattern, "zzz");

Result:

azzzc

It’s very slow though. Increasing the number of options to 10 million gives me an OutOfMemoryException.

You probably would benefit from looking at another approach.