Need Regular Expression that Validate Decimal (18,3)

Need Regular Expression that Validate Decimal (18,3)

first thanks for helping in advanced :D , i need a Regular that validate
Decimal (18,3) this mean max number of digits before comma is 15 and
accept 3 numbers after comma . 18 precision 3 scale .
Valid
123456789.123
123456789123456.12
12345.1
123456789123456
not valid
1234567891234567
123.1234
1.12345
.1234
Thank you .