समानस्य छन्दस्यमूर्धप्रभृत्युदर्केषु

Operational rule vidhi

स is substituted for समान् in the छन्दस् (Vedas), but not before 1. मूर्धन् 2. प्रभृति and 3. उदर्क.

Word by word (padaccheda)

Target समानस्य ṣaṣṭhī · ekavacanaRight context छन्दसि saptamī · ekavacanaRight context अमूर्ध- प्रभृति- उत्- अर्केषु saptamī · bahuvacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").

Full reading

समानस्य छन्दसि अमूर्ध-प्रभृति-उत्-अर्केषु

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

Commentary

स is substituted for समान् in the छन्दस् (Vedas), but not before 1. मूर्धन् 2. प्रभृति and 3. उदर्क।
Thus अनुभ्राता सगर्भ्यः, अनुसखा सयूथ्यः, योनः सनूत्यः (See 4.4.14). समानो गर्भः = सगर्भः, तत्र भवः = सगर्भ्यः formed by यन् affix 4.4.114. But समानमूर्द्धा, समानप्रभृतयः, समानोदर्काः ॥ This substitution takes place in secular literature also, as सपक्षः, साधर्म्यम्, सजातीयः &c.

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