How to prevent text replacement from being triggered by alphanumeric characters?
When I'm using Messenger on Chrome, this triggers text replacement and becomes &quot;2 like-you arei&quot;. This doesn't happen on StackExchange, but it does happen on Messenger.

Text Replacements work on apps that utilize the Cocoa interface.  This means any Apple application and any third party app like Chrome, will have text Replacements compatability.
To better control the replacement triggering as  well as gain functionality whether or not you're using a Cocoa based app, you need an an app like Text Expander ($3.33/mo) or aText ($5).   Both can be configured to use a short cut, a hot hey, or  delimiter (like Tab) to trigger it.  So in your example, you'd have to type &quot;ur&quot; followed by Tab for it to trigger.  Firefox, however doesn't  support it.
An alternate method would be to put a special character in front of your replacement pattern like a tilde (~) or a an exclamation  mark (!) as its highly unlikely you'll ever type that sequence of characters as a &quot;normal&quot; sequence.  This has the advantage of being free, but you don't get the increased functionality of the 3rd party apps like customized triggers and universal availability (not limited to Cocoa based apps)