The rule for the sesting of the regex type I get from here. I create the small demo which use regex:/^(foo|bar)$/ in the 'Client' column. Only two values will be valide "foo" and "bar". To go in the inline editing mode one should use double.click.
The most problem I see in $.jgrid.edit.msg.regex string which should be localized, but http://translate.google.com can do produce the first version of the translations.
By the way if would be very nice to modify lines 131-136 of grid.inlinedit.js to permit more customization of the validation errors.
I hope that regex could reduce the requirement to use of custom edit rule.
I agree with you that one can implement all with respect of custom verification rule, but the approach has some disadvantages.
If somebody is new in jqGrid he try to understand and use standard "predefined" features. It is good if he find out that formatter are exist. Then he look through the sandard verification rules and use one from there. Less then 30% of the user will use custom verification rule. So if you think from the point of view of new or not advanced users you will see that predefined regex formatter can has some advantages.
The feature is of course not very important, it is more cosmetical nature and just want to makes some thing to be used easier. I suggested this only after have seen some questions which can be easy solved by the formatter. It seems to me that at least 90% of custom verification rules do just test for regex.
I am not sure, Tony what is the best way. Probably you are right, but on the other side if you compare the problem with the same problem of the validation plugin you will see that many people find interesting the question. They also missing the regular expression validator and find it practical (24 reputation points for the question and 48 reputation points for the solution).