Java remove non numeric characters from string except x

use this: [^x0-9]

You may check it on http://gskinner.com/RegExr/

Your regex is

number.replaceAll("[^\\dxX]+", "");

No need to escape normal characters inside a character class. An improvement is also to have the quantifier + after the character class, that way sequences of those characters are replaced at once and not each char on its own.

Read some regex basics on Xisb: What absolutely every Programmer should know about regular expressions