Interestingly, Regan has been used for more boys than girls in all of the last 20 years, according to ONS data. I thought it was a girl's name too (I've only known one in my entire life, and she is a woman), but boys heavily outweigh girls.
Last year 111 boys were named Regan (the 395th most popular boys name) and only 14 girls were given the name (the 2008th most popular girls name).