"\K" or "?<=" don't seem to work.

Hello and Happy New Year. This problem was initiated on stackoverflow, and it seems that \K (capital K), or ?<= (which, in the *.lang file it needs to be written as ?<=) don't seem to work. Here's the two version that work on regex101 site, but not in the *.lang file:

<context id="variable-noequal" style-ref="variable">
  <match>variab(le|el)\s+\K([_a-zA-Z][_a-zA-Z0-9]*)\s</match>
</context>`

<context id="variable-noequal" style-ref="variable">
  <match>(?&lt;=(variab(le|el)\s+))([_a-zA-Z][_a-zA-Z0-9]*)\s</match>
</context>`

The first one has no effect, the second one issues a lookbehind assertion is not fixed length. The additional proposal in the comments of the stackoverflow post doesn't work, either (or combinations with the hammer). Am I doing something wrong or is this a known limitation? If the latter, is there a workaround?

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Forgot to add: I'm using Archlinux x64, gEdit 3.30.2. The problem is also in Mousepad, Pluma, Xed (I suppose it's to be expected?).