how to check character in field

I have table having maximum number values in character datatype. I want to find those values where no character is defined and replace it with null. eg. table having field receiptno which is character field. I have minimum records having values like a12455,c24353,21243,c34345.
I want to replace these values with null or 0 and change this field with number datatype.

How can I write case statement for this field in simple sql query?????
Please help.

receiptno
a12345
12234
122434
23255
d34355

I want output like

0
12345
122434
23255
0