Get Numerical value on a sign

Is it possible to take a line of a billboard, but what is the numerical value? To not take the letters? Because if I put "XP: {XP}", so I can not subtract from the person the number of level marked on the line because the line takes "XP:" with.
Or take the value of the line before the plugin adds the "XP:"

 

Need more context.
Are you trying to levelup a player based on what number is on a sign?

 

For an enchantment plugin by panels with xp.

 

up please