Hindi–Urdu transliteration
Hindi–Urdu (Devanagari: हिन्दी-उर्दू, Nastaliq: ہندی-اردو) (also known as Hindustani) is the lingua franca of modern-day Northern India and Pakistan (together classically known as Hindustan). Modern Standard Hindi is officially registered in India as a standard written using the Devanagari script, and Standard Urdu is officially registered in Pakistan as a standard written using an extended Perso-Arabic script.
Hindi–Urdu transliteration (or Hindustani transliteration) is essential for Hindustani speakers to understand each other's text, and it is especially important considering that the underlying language of both the Hindi & Urdu registers are almost the same. Transliteration is theoretically possible because of the common Hindustani phonology underlying Hindi-Urdu. In the present day, the Hindustani language is seen as a unifying language, as initially proposed by Mahatma Gandhi to resolve the Hindi–Urdu controversy. ("Hindustani" is not to be confused with followers of Hinduism, as 'Hindu' in Persian means 'Indo')
Technically, a direct one-to-one script mapping or rule-based lossless transliteration of Hindi-Urdu is not possible, majorly since Hindi is written in an abugida script and Urdu is written in an abjad script, and also because of other constraints like multiple similar characters from Perso-Arabic mapping onto a single character in Devanagari. However, there have been dictionary-based mapping attempts which have yielded very high accuracy, providing near-to-perfect transliterations. For literary domains, a mere transliteration between Hindi-Urdu will not suffice as formal Hindi is more inclined towards Sanskrit vocabulary whereas formal Urdu is more inclined towards Persian and Arabic vocabulary; hence a system combining transliteration and translation would be necessary for such cases.
In addition to Hindi-Urdu, there have been attempts to design Indo-Pakistani transliteration systems for digraphic languages like Sindhi (written in extended Perso-Arabic in Sindh of Pakistan and in Devanagari by Sindhis in partitioned India), Punjabi (written in Gurmukhi in East Punjab and Shahmukhi in West Punjab), Saraiki (written in extended-Shahmukhi script in Saraikistan and unofficially in Sindhi-Devanagari script in India) and Kashmiri (written in extended Perso-Arabic by Kashmiri Muslims and extended-Devanagari by Kashmiri Hindus).