HTML tag validation added for tag names

[akhilgkrishnan]

Follow-up #49120, Also fixes the empty string "" case of #44948

Motivation / Background

As per the discussion in #49120 (comment), finally decided to have validation for invalid HTML characters in tag, content_tag helper methods.

Detail

Added validation for HTML tag names in the tag and content_tag method. The content_tag method now checks that the provided tag name adheres to the HTML specification. If an invalid HTML tag name is provided, the method raises an ArgumentError with an appropriate error message.

Examples:
Before:

content_tag("12p") 
#  "<12p></12p>"

content_tag("") 
# "<></>"

tag("image file")
#  "<image_file></image_file>"

After:

# Raises ArgumentError: Invalid HTML5 tag name: 12p
content_tag("12p") # Starting with a number

# Raises ArgumentError: Invalid HTML5 tag name:  
content_tag("") # Empty tag name

# Raises ArgumentError: Invalid HTML5 tag name: "image file"
tag("image file") # Contains a space

cc\ @byroot

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[byroot]

Looks fine to me now, the only outstanding thing is what to allow exactly.

The HTML5 spec is fairly restrictive:

Tags contain a tag name, giving the element's name. HTML elements all have names that only use characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z, and U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z. In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive.

https://www.w3.org/TR/2011/WD-html5-20110405/syntax.html#syntax-tag-name

But I don't know if perhaps there is another later spec that allow more. I'd hate to ship something like this and to break the code of people using some less known part of the spec.

[akhilgkrishnan]

Looks fine to me now, the only outstanding thing is what to allow exactly.

The HTML5 spec is fairly restrictive:

Tags contain a tag name, giving the element's name. HTML elements all have names that only use characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z, and U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z. In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive.

https://www.w3.org/TR/2011/WD-html5-20110405/syntax.html#syntax-tag-name

But I don't know if perhaps there is another later spec that allow more. I'd hate to ship something like this and to break the code of people using some less known part of the spec.

As per the current HTML restriction, the implemented regex validates all these.

[akhilgkrishnan]

@byroot Can we proceed with this?

[flavorjones]

@akhilgkrishnan I see you requested a re-review, but I don't see any changes to the PR. Help me understand what you'd like me to give feedback on?

[akhilgkrishnan]

@akhilgkrishnan I see you requested a re-review, but I don't see any changes to the PR. Help me understand what you'd like me to give feedback on?

Can I get your opinion on this? #49175 (comment)

[akhilgkrishnan]

@flavorjones Let me know if any changes required here.

cc: @byroot

[flavorjones]

Other than the slight tweak to the exception message, this looks good. Thank you for taking the time to work with me on it!

[akhilgkrishnan]

Other than the slight tweak to the exception message, this looks good. Thank you for taking the time to work with me on it!

Thanks for your feedback and support for completing this. ❤️