In Standard Italian, in most cases, in a word beginning by “r” + vowel, the “r” is pronounced as a simple, non-geminate one.
The “most cases” include the word in isolation, the word preceded by a consonant and, in the majority of the cases, the word preceded by a vowel. So, regalo, un regalo, quattro regali all are pronounced with a simple “r”.
The exception comes from a phenomenon called raddoppiamento sintattico (or fonosintattico): in Standard Italian some words ending by a vowel, followed by a word beginning by a consonant, cause a gemination (doubling) in that consonant. These words are those with two or more syllables in which the last one is stressed (so, paltò rosso is pronounced as if it were written paltorrosso), some monosyllables (e, è, se, a, da and other ones: a Roma sounds like arròma) and a few two-syllable words (come, dove, sopra, qualche).
If you read Italian, you can find more in the article quoted above.
All of this holds for Standard Italian: various regional varieties have small or large differences from this, ranging from the choice of the words that induce the raddoppiamento (for instance, in Rome most of the above holds, but da doesn't geminate the consonant that follows), to extending the doubling to many more cases or, vice versa, speaking with almost no geminate consonants.