How to use regex with vega

I am new to vega lite / vega.

I'm trying to write a regex query for a java log error as following with vega tree layout.
A field 'message' contains :

[Poolthread] com.xxxx.content.core-bundle com.xxxxx.content.model.impl.RegisterTypeInternal(3179)] The activate method has thrown an exception (com.xxxxx.content.model.exception.ModelException: ModelException: {Code}-LCC-REP-FCT-002, {Message}-Access denied)
com.xxxx.content.model.exception.ModelException: ModelException: {Code}-LCC-REP-FCT-002, {Message}-Access denied
	at com.xxxxx.content.repository.utils.ExceptionUtil.getException(ExceptionUtil.java:52)
	at com.xxxxx.content.repository.utils.ExceptionUtil.getException(ExceptionUtil.java:171)

i'm use this regex which gets an exception word before the character : (like as ModelException ) but i don't how use this regex in vega . i tested this regex on the site regex101, you can test it this url [regex101: build, test, and debug regex](https://regex101 test)
i would that regex return value of the first group 1 : ModelException .

([a-zA-Z0-9_]+)(?=:)

But the regexp functions return only boolean values. This function is used with the split and replace function. I can't use to define group, find, matchers like in java programming. I just wanted it to return the value found . otherwise if you have examples of the regexp functioon with vega. i want to .

Thank for your help

If you are looking to replace a string using a regex pattern then you would use the replace function and define the regex pattern there.

no I just wanted regex to return the match found as if I extract a word from the message

I would look for a way to do this via DSL query against the index. Don't believe Vega has this functionality.

I wanted to see if there is an alternative