for Regexp validator set html input tag 'pattern' attribute

[lefty01]

for Regexp validator set html input tag 'pattern' attribute (used if novalidate not set on a form)

Similar to the Length validator that sets the html input attributes minlength and maxlength I found it useful to have this for regex patterns as well.
One advantage of using the browser is to get the message translated already (even tough here it is a bit generic, since it only says input needs to match the requested format and does not tell about the format itself)

added some basic test ... not sure if that is accepted this way?

[azmeuk]

Thank you for you contribution.
I think an additional test that actually generate some HTML would be good to have, maybe in the fashion of https://github.com/wtforms/wtforms/blob/7418bac10d689c8ca428e66d465317d9fcd7b215/tests/fields/test_html5.py#L148

Also, can you add a CHANGELOG entry?

For the record, the related doc : https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/pattern

[azmeuk]

I am not sure of the point of this test. I suppose the assertion could go on test_regex_passes.

[lefty01]

... need to check again .. I was not that sure about proper testing here. will take a closer look Thx!

[azmeuk]

From the MDN documentation.

The pattern attribute is an attribute of the text, tel, email, url, password, and search input types.

You would probably want to add the pattern flag to the concerned widgets too.

[azmeuk]

I was thinking that python and JS regex might have different features: see here or here.

In that case, a valid python regex might not be a valid JS regex. There are compatibility layers like https://pypi.org/project/js-regex/ (but this one is unmaintained).

As differences mainly concerns advanced regex, i am not sure this is a problem.

[azmeuk]

@lefty01 are you interested in finishing the PR or would you like to let someone else take it over?

[ThiefMaster]

IMHO this should be configurable. You might not always want to send the pattern to the client, or you may want to use regex features not supported in the browser.

[lefty01]

@lefty01 are you interested in finishing the PR or would you like to let someone else take it over?

well, interested yes of course but right now it's not on top of my list ;)
so I'm quite happy there's now further discussion ongoing and of course do not mind at all if you/someone takes over the item so we can close it at some point.
... been a while and its bit hard to follow up anyways ;)

[dougthor42]

FYI: In my experience with HTML pattern, there are two caveats:

1. I've found that the HTML pattern string needs to be slightly different than a standard python re.pattern string. IME the difference is typcially related to escaping.
   • Example: Python accepts r"[-\w]*", but HTML pattern must be r"[\-\w]*"
2. It doesn't seem possible to "clean" a verbose regex back to a bare pattern string, so any patterns that are passed to HTML pattern can't be verbose. See https://stackoverflow.com/q/35640871/1354930