I’d recommend using .NET Regex Tester - Regex Storm instead of other regex builders, since it is specifically made for vb.net
The following regex worked fine for me to find a 17-digit number starting with 21 or 11. Note that I added \b before & after the digits because I assumed you wouldn’t want a 18+ digit found.
\b(21|11)\d{15}\b
Anil_G
ashokkarale
Ajay_Mishra
Gautham_Pattabiraman
BHUSHAN_NAGAONKAR1
vrdabberu
ABHIMANYU_THITE1
lrtetala
samantha_shah
shyamala_shyamu