This is just a quick note to discuss something interesting a reader and I encountered last week. As you know (hopefully!), the input tag supports a pattern argument. The value of the pattern argument is a regular expression that is compared against the value of the input field. This allows for custom types of validation for data not covered by the host of new field types added in HTML5.
But here’s an interesting question. What exactly happens when you supply a bad regular expression? As you might expect – absolutely nothing. A reader on my blog noticed that a pattern value he had used was letting everything pass and not validating as he expected. When this happens, nothing will show up in the console and you really don’t have a good idea as to what is going on.
I opened up my console, took his regex, and simply put it in a new RegExp object. The second I pasted in his bad regex, I got an error. Here is an example:
Something to keep in mind, folks!