The page about placeholders on the wiki (https://github.com/xmlunit/user-guide/wiki/Placeholders) describes an example placeholder isNumber

The implementation of this placeholder should:

• work on attribute and element values (in the same way that ignore does)
• allow the diff engine to identify differences if the attribute or element value cannot be parsed as a number
• acceptable numbers include positive and negative integers, positive and negative floating point numbers, and positive and negative floating point numbers in engineering notation