I know that sounds vague. Basically I just want Sublime to highlight custom syntax (color the text), just like it does with native syntax.
I am using Sublime to write LaTeX code. For those that don't know, LaTeX equations are typically enclosed by \[ \], e.g.
\[ E = m c^2 \]
Sublime understands that syntax and colors the enclosing code appropriately.
However, I use my custom defined command, \eq{ ... }, which wraps the \[ \] functionality (so I can globally change some settings by just redefining the \eq definition). e.g.
\eq{ E = m c^2 }
I don't know anything about Sublime under the hood beyond basic key bindings. I want to expand Sublime's understanding of syntax to incorporate my custom command without wasting a ton of time digging through tutorials and such.
Since you are mainly interested in the result and not in the reasoning, I will try to be as straight forward as I can.
The LaTeX syntax of Sublime Text will change in release 3119 and I would recommend to use that, if you want to change something.
Just download it from https://github.com/sublimehq/Packages and put the LaTeX folder into the folder, which opens when you select Preferences >> Browse Packages... in the Sublime Text menu.
Afterwards open the file LaTeX.sublime-syntax and search for ensuremath (LaTeX.sublime-syntax#L498). Duplicate that part (everything with a higher indent) and change the command to the command you wish, e.g. in your example this would be - match: '((\\)eq)(\{)'.
Aside the new syntax removes the highlighting of math environments as strings, because this has lead to several problems. I made a small entry in the LaTeXTools wiki to explain, how you restore the highlight.