advanced regex pattern for html5 input

Question

I can't seem to put together a working pattern to disallow all html tags except for the strong and em tag.

I don't want to parse the html but just want to give the user a warning that the input will not be accepted. I am aware that this is not supported in all browsers but I would love a pure html solution, as I already have a working JS solution, but I wan't to layer the user experience.

<input name="user_input" pattern="^(?!<[^>]*>).*$" />

So allowed tags: strong, em the use of all other tags should make the result false

Any one able to crack this one?

KR

edit:

<input type="text" pattern="((?!<(?!\/?(strong|em))[^>]*>).)*">

is what seems to do the trick. Thank you for your help!

Answer 1

You can use a Negative Lookahead (?!) for this purpose.

An example regex string which matches the entire pair:

<(?!\\/?strong|\\/?em)[^>]*>.*(?:<\\/.*?>)?

A shorter regex, which matches the first tag only

<(?!\\/?(strong|em))[^>]*>

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This match will pass if a HTML tag with something EXCEPT strong or em exists.

So, if match = $true, you can deny the input and give the user a warning.

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Regex101 demo