Matilda de Braose was married to Rhys Mechyll, son of Rhys Gryg and grandson of the Lord Rhys. After the death of her husband in 1244 she placed the castle of Carreg Cennen in the hands of the Normans. (The pictures below show the remains of the later 13th/14th century castle on the same impressive site.)
In 1248, according to the Brut y Tywysogyon, "Rhys Fychan ap Rhys Mechyll regained the castle of Carreg Cennen, which his mother had treacherously placed in the power of the French, out of enmity for her son."
I have concluded tentatively that the only candidate for the identity of the Matilda de Braose who married Rhys Mechyll is Reginald's daughter, given the close relationships between Reginald and Rhys' father at the time. (Though I admit the evidence that he had a daughter Matilda does
not come readily to hand!)
Father: Reginald de Braose
Mother: Grace, daughter of William Brewer
Matilda was married to Rhys Mechyll (of Deheubarth) d.1244
Child 1: Rhys Fychan (or Ieuanc) d.1271