c# - Would a regex lookaround work to match one or two numbers? -

i taking in user input (string) , checking against 3 different reg ex patterns. reason doing asynchronous validation , validation function firing on every key up. user input string concatenations, longer each iteration.

the regex should match 1 3 digit numbers, space, again 1 3 digit numbers. examples:

correct:  1  12 12 26 123 12  incorrect letter or spec character space before number (space after first set of numbers ok). 

the following function works:

    static void main(string[] args)     {         string userinput = console.readline();          list<string> patternlist = new list<string>();         patternlist.add(@"^([0-9])$");         patternlist.add(@"^[0-9]{1,3} $");         patternlist.add(@"^[0-9]{1,3} [0-9]{1,3}$");          validateinput(userinput, patternlist);     }      private static void validateinput(string userinput, list<string> patternlist)     {         regex r = null;         match m = null;         bool ismatch = false;         foreach (var p in patternlist)         {             r = new regex(p);             m = r.match(userinput);             ismatch = m.success;             if (ismatch)                 break;         }                                 if (ismatch)         {             console.writeline("you wrote: {0} thats match", userinput);             console.readline();         }         else         {             console.writeline("you wrote: {0} thats not match", userinput);             console.readline();         }     } 

but looking see if 1 expression (instead of 3) can used. lookarounds, negative behind later in expression make sure there number preceding space?

you don't need lookaround @ all, can compress 3 regexes single one:

^([0-9]{1,3})(?: ([0-9]{1,3})?)?$ 

?: means non-capture group, first matching group capture first number, , second capture group evidently second.

in case space sequence between 2 numbers arbitrary (one or more spaces, tabs, whatever), can use:

^([0-9]{1,3})(?:\s+([0-9]{1,3})?)?$ 

instead.

lookahead , lookbehind can evidently used, making things way more complicated , harder parse. furthermore, lookahead in many cases used and pattern, not or pattern. or pattern can implemented disjunction (|).