How to use variables with regex?

Question

This is the input string: 23x^45*y or 2x^2 or y^4*x^3.

I am matching ^[0-9]+ after letter x. In other words I am matching

Answer 1

You can use the pattern @"[cEle]\^\d+" which you can create dynamically from your character array:

string s = "23x^45*y or 2x^2 or y^4*x^3";
char[] letters = { 'e', 'x', 'L' };
string regex = string.Format(@"[{0}]\^\d+",
    Regex.Escape(new string(letters)));
foreach (Match match in Regex.Matches(s, regex))
    Console.WriteLine(match);

Result:

x^45
x^2
x^3

A few things to note:

• It is necessary to escape the ^ inside the regular expression otherwise it has a special meaning "start of line".
• It is a good idea to use Regex.Escape when inserting literal strings from a user into a regular expression, to avoid that any characters they type get misinterpreted as special characters.
• This will also match the x from the end of variables with longer names like tax^2. This can be avoided by requiring a word boundary (\b).
• If you write x^1 as just x then this regular expression will not match it. This can be fixed by using (\^\d+)?.

Answer 2

Try using this pattern for capturing the number but excluding the x^ prefix:

(?<=x\^)[0-9]+

string strInput = "23x^45*y or 2x^2 or y^4*x^3";
foreach (Match match in Regex.Matches(strInput, @"(?<=x\^)[0-9]+"))
    Console.WriteLine(match);

This should print :

45
2
3

Do not forget to use the option IgnoreCase for matching, if required.