Question

In my web application, I create some framework that use to bind model data to control on page. Each model property has some rule like string length, not null and regular expression. Before submit page, framework validate any binded control with defined rules.

So, I want to detect what character that is allowed in each regular expression rule like the following example.

"^[0-9]+$" allow only digit characters like 1, 2, 3.
"^[a-zA-Z_][a-zA-Z_\-0-9]+$" allow only a-z, - and _ characters

However, this function should not care about grouping, positioning of allowed character. It just tells about possible characters only.

Do you have any idea for creating this function?

PS. I know it easy to create specified function like numeric only for allowing only digit characters. But I need share/reuse same piece of code both data tier(contains all model validator) and UI tier without modify anything.

Thanks

Answer 1

You can’t solve this for the general case. Regexps don’t generally ‘fail’ at a particular character, they just get to a point where they can’t match any more, and have to backtrack to try another method of matching.

One could make a regex implementation that remembered which was the farthest it managed to match before backtracking, but most implementations don’t do that, including JavaScript’s.

A possible way forward would be to match first against ^pattern$, and if that failed match against ^pattern without the end-anchor. This would be more likely to give you some sort of match of the left hand part of the string, so you could count how many characters were in the match, and say the following character was ‘invalid’. For more complicated regexps this would be misleading, but it would certainly work for the simple cases like [a-zA-Z0-9_]+.