Speaking of which, I'm curious to see resulting regional populations.
So I'll do it here as fast as I can by following the latest INSEE data (2012)...
Sorted descending.
Île-de-France (Paris region) => 11,898,502
Auvergne (1,354,104) + Rhône-Alpes (6,341,160) => 7,695,264
Nord-Pas-de-Calais (4,050,756) + Picardie (1,922,342) => 5,973,098
Aquitaine (3,285,970) + Limousin (738,633) + Poitou-Charentes (1,783,991) => 5,808,594
Languedoc-Roussillon (2,700,266) + Midi-Pyrénées (2,926,592) => 5,626,858
Alsace (1,859,869) + Lorraine (2,349,816) + Champagne-Ardenne (1,339,270) => 5,548,955
Provence-Alpes-Côte d'Azur => 4,935,576
Basse-Normandie (1,477,209) + Haute-Normandie (1,845,547) => 3,322,756
Pays de la Loire => 3,632,614
Bretagne => 3,237,097
Bourgogne (1,641,130) + Franche-Comté (1,175,684) => 2,816,814
Centre(-Val de Loire) => 2,563,586
Réunion (overseas region) => 833,944
Guadeloupe (overseas region) => 403,314
Martinique (overseas region) => 388,364
Corse => 316,257
Guyane (overseas region) => 239,648
Mayotte (overseas region) => 212,645