If the goal is to make it denote "inserted text", I think it should be visually different to "underline" - something more similar to the green we use for actual diff views, maybe more subtle. Will need special handling in the color-blindness themes.

Digging deeper, the problem is specific to ProjectRemarkupRule::getObjectIDPattern. That returns:

Step 2: Remove irrelevant character class exclusions:

Disentangling that monster regex, step 1:

Indeed https://devina.io/redos-checker says the aformentioned regex is exponential time worst-case

I monkeypatched the code to print preg_last_error when the problematic preg_match returns null. The error is "Backtrack limit exhausted".

This exception happens once $rule in the loop foreach ($this->getMarkupRules() as $rule) in PhutilRemarkupBlockRule::applyRules($text) becomes ProjectRemarkupRule. That's where it blows up.

Taking the best from the two: what about assuming a default @link /book/group/best-document-ever-about-link - but still allowing custom @link to allow easy migrations?

This is an issue in https://we.phorge.it/source/phorge/browse/master/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php . In getObjectEmbedPattern(), preg_quote($prefix) returns \# and the method finally returns (\B{\#([^.\s?!,:;{}#\(\)"'\*/~]+(?:[^\s?!,:;{}#\(\)"'\*/~]*[^.\s?!,:;{}#\(\)"'\*/~]+)*)([,\s](?:[^}\\]|\\.)*)?}\B)u.
Within apply($text), $text becomes null after that first preg_replace_callback, so the second call barks.

I think GitHub allows that syntax only in comments 🤔

In T15744#18236, @20after4 wrote:

I was unaware of `#0969DA` syntax from github/gitlab.

I was unaware of #0969DA syntax from github/gitlab. I'm not sure if I like that syntax better than {} but I am generally in favor of using the same syntax as other systems in order to converge towards defacto standardization.