Matching result not same as glibc regex

[Hengliy]

Hi， I test tre-regex comparted with glibc-regex and have some discover：

text = "a"
pattern = "(.?).?\1"

glibc-regex:

matching: "a"
pmatch[0].rm_so = 0
pmatch[0].rm_eo = 1

matching: ""
pmatch[1].rm_so = 0
pmatch[1].rm_eo = 0

tre-regex:

matching: ""
pmatch[0].rm_so = 0
pmatch[0].rm_eo = 0

matching: ""
pmatch[1].rm_so = 0
pmatch[1].rm_eo = 0

Is this a bug？

Thanks.