^[+]?[\d]{1,3}[\-]?(([\d]{8,18})|([(]{1}[\d]{8,18}[)]{1}))
Lets explain this RegExp part by part.
"^[+]?" this part ensures that there will be an optional first character which will be a "+". And if you will start with a digit then it will be fine due to the next part of Expression.
"[\d]{1,3}" this part will ensure that you must enter a number not more than 3 digit and not less than 1 digit.
"[\-]?" this part will ensure that you may enter a "-"after satisfying first two part.
"[\d]{8,18}" this part ensure that you need to enter a number from 8-18 digit.
"[(]{1}[\d]{8,18}[)]{1}" this part will ensure that you will start with a "(" and enter a number between 8 to 18 digits and at the end you will enter a ")".
Now "(([\d]{8,18})|([(]{1}[\d]{8,18}[)]{1}))" this part will ensure to have any one part of these two to be satisfied.
