@some examples of ru-rules

Words with true r (like punar पुनर्) follow the normal rules of sound change:

**punar पुनर् + **dravati द्रवति
→ !**punardravati पुनर्द्रवति "runs again" jbj

**punar पुनर् + **uvAca उवाच
→ !**punaruvAca पुनरुवाच "talked again" jbj

ahar अहर् + dIrgham दीर्घम्
→ ahardIrgham अहर्दीर्घम् "a long day" jbj

punar पुनर् + pause
→ punaH पुनः by kharavasAn...

punar पुनर् + tatra तत्र
→ punaH tatra पुनः तत्र by kharavasAn...
→ punas tatra पुनस्तत्र "it's there again", by visarjanIy...

Words ending in s स्, however, are caught by the special rule sasajuSoruH, which turns their s स् into a "ru". This /ru thing works like a normal r र् more often than not:

azvas अश्वस् + pause
→ azva अश्व + /ru + pause by sasajuSoruH
→ azvaH अश्वः by kharavasAn...
gurus गुरुस् + pause
→ guruH गुरुः by the same rules
azvas अश्वस् + tatra तत्र
→ azva अश्व + /ru + tatra तत्र by sasajuSoruH
→ azvaH tatra अश्वः तत्र by kharavasAn...
→ azvas tatra अश्वस्तत्र "the horse is there" by visarjanIy...

However, in some cases, the /ru is caught by some special /ru-rules that will work on a /ru but not on a true r. One such rule is hazica —

azvas अश्वस् + dravati द्रवति
→ azva अश्व + /ru + dravati द्रवति by sasajuSoruH
→ azva अश्व + u उ + dravati द्रवति by hazica
→ azvo dravati अश्वो द्रवति by AdguNaH

mArgas मार्गस् + dIrghas दीर्घस् + pause → mArgo dIrghaH मार्गो दीर्घः

(See **azvotra अश्वोत्र for another example, this time with the special /ru-rule atororapl....)

The rules for /ru and for true r always give the same result if the r र् s स् is not before /sup' (see rule rossupi) and is not after a अ A आ.

Incidentally, words that end in r र् are seldomer than green dogs. See list of common words ending in true r for some examples.