c# - Multiplication with .NET regular expressions -

in spirit of polygenelubricants' efforts silly things regular expressions, try .net regex engine multiplicate me.

this has, of course, no practical value , meant purely theoretical exercise.

so far, i've arrived @ monster, should check if number of 1s multiplied number of 2s equals number of 3s in string.

regex regex = new regex( @" ^ (1(?<a>))*  # increment each 1 (2(?<b>))*  # increment b each 2     (?(a)   # if > 0         (                                (?<-a>)             # decrement             (3(?<c-b>))*        # match 3's, decrementing b , incrementing c until                                 # there no 3's left or b 0             (?(b)(?!))          # if b != 0, fail             (?<b-c>)*           # b = c, c = 0         )     )*      # repeat (?(a)(?!))  # if != 0, fail (?(c)(?!))  # if c != 0, fail $ ", regexoptions.ignorepatternwhitespace); 

unfortunately, not working, , @ loss why. commented show think engine should doing, may off here. examples of output:

regex.ismatch("123") // true, correct regex.ismatch("22") // true, correct regex.ismatch("12233") // false, incorrect regex.ismatch("11233"); // true, correct 

any thought welcome!

i'm pretty sure problem in line:

(?<b-c>)* 

from can tell, no text match in there, regex refuses match more 1 time. slimmed down regex following:

(1(?<a>))* (?(a)(?<-a>))* (?(a)(?!)) 

which passes on 1 fails on 111. tried (?<-a>)*. no difference. however, changing to

(1(?<a>))* (?(a)((?<-a>)(2(?<b>))(?<-b>)))* (?(a)(?!)) 

passes on both 12 , 111222. going match of "" match causes regex work expected.

getting original regex, guess (?<b-c>)* matching 0-1 times, explains why having 1 2 in string works, having more 1 fails.

using string of 11 fails, follows same logic, makes entire match "", means matches once, causing (?(a)(?!)) fail.