पश्च पश्चा च च्छन्दसि

Operational rule vidhi

The words पश्च and पश्चा are anomalous in the छन्दस् (Vedas) having the force of अस्ताति.

Word by word (padaccheda)

Indeclinable पश्च avyayaIndeclinable पश्चा avyayaIndeclinable च avyayaRight context छन्दसि saptamī · ekavacana

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

पश्च पश्चा च छन्दसि

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

Commentary

The words पश्च and पश्चा are anomalous in the छन्दस् (Vedas) having the force of अस्ताति।
By च, the word पश्चात् is also included. As पुरा व्याघ्रो जायते, पश्च, पश्चा or पश्चात् सिंहः ॥ As प॒श्चात् पु॒रस्ता॑दध॒रादुद॑क्तात् (Rig Ved X. 87.21) प॒श्चेदम॒न्यद॑भव॒द्यज्ज॑त्रम् (Rig. Veda. X. 149.3).

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