Question

I’m trying to get the pipe (OR) to work. I have this regular expression with these tests:

   string output = "test: {0} matches";
   string test = "AND [Field] =";

   Regex r = new Regex(@"AND|OR\s[?\w+]?\s?=");

   if (r.IsMatch(test)) 
      Console.WriteLine(string.Format(output, test));

   test = "OR [Field] =";
   if (r.IsMatch(test))
      Console.WriteLine(string.Format(output, test));

The first match passes but the second fails. I can’t figure out why the | between the AND and OR does not act like an OR statement. I’ve tried putting parenthesis around the AND|OR but then both test fails.

Can anyone help me with the OR “|” statement please?

Answer 1

Within regex, alternation (|) has lower precedence than sequence. Your regex pattern is interpreted as “AND or (OR followed by \s[?\w+]?\s?=)” (note the parenthesis I added to denote precedence).

To achieve what you want, you’ll need to parenthesize your AND|OR part, as well as escape your [ and ] characters.

Regex r = new Regex(@"(AND|OR)\s\[?\w+\]?\s?=");

Furthermore, you probably do not want to allow an [ without a ] (or vice versa); either both should be present, or neither:

Regex r = new Regex(@"(AND|OR)\s(\[\w+\]|\w+)\s?=");