Multiline post content with `<` as first character becomes malformed

[johnjamesjacoby]

Create a page/post with the following content:

< Hello
< Hello
< Hello

View the post, and post_content becomes:

< Hello < Hello < Hello

[johnjamesjacoby]

I've narrowed it down to:

• wpautop()
• wp_replace_in_html_tags()
• wp_html_split()
• get_html_split_regex()

Something is up with how get_html_split_regex() identifies HTML tags.

---

< 1
< 2
< 3

gets treated differently than:

> 1
> 2
> 3

[soulseekah]

That is not valid HTML. < should be used.

Related #39153