PDF In-Depth

Slicing and Dicing - Formatting user input

June 03, 2001


The question

The other day, a fellow wrote in to one of the PDF discussion forums to ask a question about formatting user input in a PDF form. It was a fairly typical question. It went something like:

Here is my problem: I am creating a form and in one field where the user enters a validation number I want the field to automatically insert thedashes between the numbers, for example: if a user inserts his numbers as 555555555 without dashes the custom Java script would automatically insertthe dashes like this 5-55-55-5555. So what I am asking is if someone could give me an example of the custom Java script that can do this.

It turns out there are many ways to solve the problem. First let's just limit our discussion to answering the question algorithmically. In other words, let's just answer the logic of the question at hand, and not worry about the messy details of trying to idiot-proof things to the point where the user can't screw things up beyond belief despite our best efforts. Idiot-proofing forms is an important consideration, but for the moment, let's sidestep it.

The slice() function

The JavaScript String class has a slice() function that retrieves substrings based on index positions that you supply as arguments. It is much easier to show how it works than to describe in words how it works, so:

/// str = "555555555"; nine fives
/// we want to end up with 5-55-55-5555
formattedStr = str.slice(0,1) + "-" +                   'str.slice(1,3) + "-" + \     

                  'str.slice(3,5) + "-" + \ 


This may not be the only or best way; it's just one way.

As for idiot-proofing: that's a whole book. My approach would be to strip all the non-numeric user input out of this particular string before beginning to format it. I.e., reduce it to a known state of just plain numbers run together. Then start putting dashes (hyphens) in, where you want them.


The real way to attack formatting of user input is, of course, to make heavy use of JavaScript's RegExp class. This is quite a complex subject and requires that you do some heavy homework. I suggest that you buy David Flanagan's JAVASCRIPT book (O'Reilly) and also Jeffrey Friedl's very fine MASTERING REGULAR EXPRESSIONS book (also by O'Reilly) if syou are serious about learning to use regexes. In the meantime, open the AForm.js file (which comes in the JavaScripts folder of your Acrobat distribution suite) with a text editor and study the many examples of RegExp method usage in that file. You'll get some good ideas there of how things work.

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