Regular Expressions in Java

https://docs.oracle.com/javase/6/docs/api/java/util/regex/Pattern.html#sum

https://www.javacodeexamples.com/java-regular-expression-remove-leading-zeros-example/104

To check it is a valid mobile or not:

A mobile number is valid if the starting digit is either 7 or 8 or 9

and must also contain 10 digits

String st="7123456789"

if(st.matches("[7-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]"))

        System.out.println("valid mobile");

(or)

if(st.matches("[7-9][0-9]{9}"))

        System.out.println("valid mobile");

(or)

if(st.matches("[789][0-9]{9}"))

        System.out.println("valid mobile");

if u want to allow 11 digit mobile number then first digit must be zero

if(st.matches("0? [7-9][0-9]{9}"))

        System.out.println("valid mobile");

0? means atmost one zero as first digit 

0 means compulsory one zero as first digit

0+ atleast one 0

0* means  0 may be there or may not be there and 0 can occur any number of times.

if u want to allow 10 or 11 or 12 digit mobile number then 

if(st.matches("(0|91)? [7-9][0-9]{9}"))

        System.out.println("valid mobile");

For email the regular expressions is

[a-z A-Z 0-9]@[a-z A-Z]. [a-z]{2-3}

=========

[^abc]  --means except abc