You need to use a regular expression that checks whether the substring of interest is at the start of the string. So, for example...
library(dplyr)
df <- data.frame(a = c("AC01Waq7", "AC2-wiggle", "AC01hithere"),
stringsAsFactors = FALSE)
print(df)
a
1 AC01Waq7
2 AC2-wiggle
3 AC01hithere
filter(df, grepl("^AC01*", a))
a
1 AC01Waq7
2 AC01hithere
Or, if you want to stay in the tidyverse...
library(stringr)
filter(df, str_detect(a, "^AC01*"))