@Random-Encounter ,
UDL is not all-powerful: it focuses on defining a list of keywords, rather than arbitrary pattern matches, for syntax highlighting, so it cannot do what you want.
However, the EnhanceAnyLexer plugin can apply the power of regex to change the foreground color of certain text; so you could install that plugin, define most of your language with the UDL interface; then, to get things to the left of an = to be a different color, use Plugins > EnhanceAnyLexer > Enhance Current Language, which will add or edit a section in its config file for your current language: the section will start with [NameOfUDLhere], and then consist of “color = regex” pairs (if the colors are in hex, it’s 0xBBGGRR for the blue, green, and red portions), along with a config line for “excluded_styles = …” which you probably don’t need to worry about)
For my example, since I’m just using the dummy “Default Langauge” (which internally is labled as udf), I can use the following config section to get it to highlight text on the left of the = with brownish-orange:
[udf]
0x66ad1 = ^.*(?=\=)
[image: 1673718642081-42434862-d71d-4824-920a-b839722f7d37-image.png]