परेश्च घाङ्कयोः

Operational rule vidhi

The र् of परि is changed to ल् before घ and अङ्क.

Word by word (padaccheda)

Target परेः ṣaṣṭhī · ekavacanaIndeclinable च avyayaRight context घाङ्कयोः saptamī · dvivacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

परेः च घाङ्कयोः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The र् of परि is changed to ल् before घ and अङ्क।
As परिघः or पलिघः, पर्यङ्कः or पल्यङ्कः ॥ The word घ here means the word-form घ, and not the technical घ of तरप् and तमप् ॥ See 3.3.84 by which हन् is replaced by घ ॥

Vart:- So also, it must be stated, before the word योगः ॥ As, परियोगः or पलियोगः ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.