पेषंवासवाहनधिषु च

Operational rule vidhi

उद is substituted for उदक before 1. पेषं 2. वास 3. वाह and 4. धि.

Word by word (padaccheda)

Right context पेषम्- वास- वाहन- धिषु saptamī · bahuvacanaIndeclinable च avyaya

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").
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Full reading

पेषम्-वास-वाहन-धिषु च

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

Commentary

उद is substituted for उदक before 1. पेषं 2. वास 3. वाह and 4. धि।
Thus उदपेषं पिनष्टि formed by णमुल् by 3.4.38; उदवासः = उदकस्यवासः, so also उदवाहनः ॥ उदकं धीयतेऽस्मिन् = उदधिः 'a water jar'.

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