syntax region end when either pattern occurs?

I am trying to syntax highlight yaml blocks in Markdown files.

Right now, I can get it so it handles either blocks that look like this:

---
author: me
document: Help
---

or

---
author: me
document: Help
...

But not both. Is there a way to do both?

I tried end=\(/^---$/\|/^\.\.\.$\) but that didn't work

unlet b:current_syntax

" Bring in YAML syntax for front matter
syntax include @Yaml syntax/yaml.vim

syntax region yamlFrontmatter start=/\%^---$/ end=/^---$/ keepend contains=@Yaml
syntax region yamlFrontmatter start=/\%^---$/ end=/^\.\.\.$/ keepend contains=@Yaml

let b:current_syntax='markdown'

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