Styjun

I have to make an assignment for Programming Techniques and I have struggled to match the free terms in a polynomial, (2.0x^2+3.5x-3.0 or 45x+2), which is given as a string. I have tried to use the following regex:

(-0,1d+(.d+)0,1)(?![0-9].*x^d))

I know the way the way I approached it might not be too good, but I did not have too much time for understanding regex in a more complex way, only slightly, and I got stuck at the point in which I would like to get only the numbers like with the format: ±# or ±#.# which are not followed by: x or x^# where # represents one or more digits.

Full regex solution :

(?<!^)[-+]?d+(.d+)?(?=$|[-+])

• 
  (?<!^) won't match if the previous character is a ^
  


• 
  [-+]? will match an optional sign character


• 
  d+(.d+)? will match the number which is composed of an integer part followed by an optional floating part


• 
  (?=$|[-+]) won't match unless what follows is a sign character or the end of the string

You can try it here.

Java solution with term-matching :

Pattern termPattern = Pattern.compile("\d+(?:\.\d+)?(x(?:\^\d+)?)?");
Matcher termMatcher = termPattern.matcher(input);
while (termMatcher.find()) 
 if (termMatcher.group(1) == null) 
 // you have a free term
 

The regex matches an integer part followed by an optional floating part then an optional x^n part which is captured in a capturing group. A Matcher is created from applying the pattern to the input. Calling Matcher.find allows us to iterate over the multiple matches on the input string. For each match we check the content of the first capturing group, when it's null we have a free term.

You can try it here.

Yet anoter solution would be to simply split the string around [+-] and for each part test whether it contains x or not.

This will do it: (?<!^)(d+(?:.d+)?)(?![x.d])

(?<! | Negative lookbehind:
 ^ | A literal "^"
) | Close group
( | Capture the following:
 d+ | Match one or more digits
 (?: | Match the following group:
 . | A literal "."
 d+ | One or more digits
 )? | Close group, optional match
) | End capture
(?! | Negative lookahead:
 [x.d] | Either an "x", ".", or any digit
) | Close group

Try it here

String s = "5x-50.1+x^2-2+20x";
Pattern p = Pattern.compile("(?<!\^)(\d+(?:\.\d+)?)(?![x.\d])");
Matcher m = p.matcher(s);
if (m.find())
 System.out.println(m.group(1)); // Output: 50.1