Resolution:
Using the below constraint in rule base file checks for minimum number of digits to 13(as an example).

&ltconstraint>
            &ltname&gtDigit Check </name>
            &ltdescription&gtMinimun digit must be 13</description>
            &ltusefor>
                   &ltvar&gtattr1</var>
           </usefor>
        &ltaction>
                &ltcheck>
                    &ltexplanation&gtMinimum Digits must be 13</explanation>
                        &ltmatch>
                            &ltvar/>
                            &ltconst type="string">/^\d{13}/</const>
                        </match>
                </check>
            </action>
</constraint>

How to have the digit check of a master catalog attribute?