Fix regex for detecting dosing compartments

[kylebaron]

See #1112 reported by @rfaelens

No need to look for leading whitespace

x <- "    F_ABS_RAPID_FORM2"
regmatches(x, regexpr("(ALAG|F|R|D)\\_[^= ]+", x, perl=TRUE))
#> [1] "F_ABS_RAPID_FORM2"

^{Created on 2023-08-24 with reprex v2.0.2}

[kylebaron]

Thanks, @kyleam; yes, will update the base.