How can I show HTML in UI

You can use use richtextdisplay function for html content...need to check for document..will keep you posted

To display an HTML document in an Appian interface, you can use an "Embedded Web Content" component.
In the interface designer, add an "Embedded Web Content" component to your interface.
Configure URL: Set the URL property of the component to the location of your HTML document. This could be a URL hosted externally or a relative URL within your organization's network.

For example, if you have a variable named htmlContent that holds your HTML code, you would configure the Rich Text Display like this:

a!richTextDisplayField(
labelPosition: "COLLAPSED",
value: ri!htmlContent /* Assuming ri!htmlContent contains your HTML code */
)

Note:docs.appian.com/.../Rich_Text_Component.html

[mention:2e70db6187af4fffafc92daf9f335d79:e9ed411860ed4f2ba0265705b8793d05]  Appian doesn't support displaying the HTML in the UI directly but there is a component available as a plugin by Appian that will be able to help you if you pass your HTML in the component's value parameter. 

[View:~/b/appmarket/posts/rich-text-editor:320:240]

I found the solution. It's very simple(Just striphtml function).  No need of regular expression or plugin hopefully.

I found the solution. It's very simple(Just striphtml function).  No need of regular expression or plugin hopefully.