Using HTML5 Input Pattern to Enforce Valid Hostname

Question

I am working with a C++ project on an ESP8266 controller and displaying a captive portal. The portal itself is created by an external library and somewhat limited with regard to the portal page generated. I only have the ability to add attributes to the form field which the code creates. In other words, I have an input field that is generated on the page/portal, and I can modify it with attributes such as pattern, min, max, etc.

I want to allow the user to enter a hostname, and to keep it exceedingly simple and compatible with potentially dated technology, I want to enforce that the hostname begins with an alpha character, must be between 3 and 24 characters, may contain alphanumeric characters and/or a hyphen, and must not end with a hyphen.

Clearly, the pattern attribute is just the thing here. The problem is I have failed so far to come up with a good working example. In my example below, it enforces a beginning alpha, and if I enter something it enforces a minimum of 3 characters, but if I enter nothing it accepts it as valid. I've also not quite figured out how to limit the last character to alphanumeric.

<input type="text" name="hostname"
    pattern="^[a-zA-Z][a-zA-Z0-9-]{2,24}$"
    title="Enter a hostname, 3-14 characters, beginning
    with a letter and containing only alphanumeric
    characters and/or a hyphen.  It must end with an
    alphanumeric."
>

So a few tests which I would like to work correctly are:

a-b (good)
a12 (good)
ab- (bad)
ab (bad)
1ab (bad)

I'm a little incredulous that my Google-fu has failed me here.

CertainPerformance · Accepted Answer · 2019-10-02 23:47:31Z

1

When you don't input anything, no validation occurs because the input is not specified as required, so add the required attribute.

Also, since you want the field to have 3-24 characters, the repetition after the first character should be {1,22}, and the last token should be [a-zA-Z\d] to ensure that the last character isn't a hyphen:

^[a-zA-Z][a-zA-Z\d-]{1,22}[a-zA-Z\d]$

<form><input type="text" name="hostname" required pattern="^[a-zA-Z][a-zA-Z\d-]{1,22}[a-zA-Z\d]$" title="Enter a hostname, 3-14 characters, beginning
    with a letter and containing only alphanumeric
    characters and/or a hyphen.  It must end with an
    alphanumeric.">
    <button>click</button>
</form>

answered Oct 2, 2019 at 23:47

CertainPerformance

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1 Comment

LCB Over a year ago

Fantastic, thank you! I was close but I wasn't gonna get that without a nudge.

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