please help me
#1
new Regex("^[0-9a-zA-Z_]{6,16}");
How to generate 0-9 a-z A-Z and "-"  rule to crack  a password 
i thank  
Hashcat   -a 3  md5.txt -2 ?l?d?u -O  --increment --increment-min 6 --increment-max 16 ?2?2?2?2?2?2?2?2?2?2?2?2?2?2?2?2
#2
Hashcat -a 3 md5.txt -2 abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_ -O --increment --increment-min 6 --increment-max 16 ?2?2?2?2?2?2?2?2?2?2?2?2?2?2?2?2
#3
Your commands are both correct. The first one is just missing the _
#4
can also simplify this with the built-in charsets. -2 ?l?u?d instead of having all values, this will accomplish the same thing.