Alternative delimiters for [link definitions][link1] are allowed -- as of
Markdown 1.0.2, I think. Hence, [this link][link2] and [this link][link3] work
too.

[link1]: http://daringfireball.net/projects/markdown/syntax#link "link syntax"
[link2]: http://daringfireball.net/projects/markdown/syntax#link 'link syntax'
[link3]: http://daringfireball.net/projects/markdown/syntax#link (link syntax)