I have used isnumeric() method to check for the strings with only numbers.
While deploying to production i am receiving the following error 'Method not found '.
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isNumeric() is a new String Method in the Winter '13 release.
Your Production instance has not yet been updated from Summer '12 (API-25) to Winter '13 (API-26) yet but your sandbox has which is why your code will not deploy.
You will need to wait till your Production instance is upgraded to deploy your code. You should have recived an email with the title 'Winter '13 Release Notification' That contains the date. (Our instance is on EU1 and is scheduled for Friday, October 5 - Saturday, October 6, 2012)
Some more Winter '13 release info for sandboxs: