UDF based on Field Length Change

Hi All,

'MATNR' field size it contains 18 characters length but when I trigger materials having more than 10 digits it throws an error.

Is there any UDF or Java code wherein if material size is '9' it should add 9 zeros in front followed by material(000000000+material).

if material size is '10' it should add 8 zeros in front followed by material(00000000+ numeric material).

if material size is '11' it should add 7 zeros in front followed by material(0000000+material).

if material size is '12' it should add 6 zeros in front followed by material(000000+material).

if material is alphanumeric it should not add any zeros in front.

Please help. Many Thanks.