Regex Tuesday Challenge - Week Fourteen

This challenge, suggested by /u/a-t-k, is to match all the valid chemical element short names (eg. H and Ag) of elements with an atomic number of 50 or less. This is golf; the aim is to create the shortest regular expression possible. You should match the chemical formulas, and nothing else.

To test a regular expression on the test cases below, type it into the text input. Each test case will be marked as passed or failed respectively - you are aiming to get as many test cases as you can to pass. Note that JavaScript must be enabled for this feature to work. The regex engine used is the JavaScript regex engine; it is similar to PCRE, but with a few differences.

Test cases (0/101)

H
match
He
match
Li
match
Be
match
B
match
C
match
N
match
O
match
F
match
Ne
match
Na
match
Mg
match
Ni
match
Si
match
P
match
S
match
Cl
match
Ar
match
K
match
Ca
match
Sc
match
Ti
match
V
match
Cr
match
Mn
match
Fe
match
Co
match
Ni
match
Cu
match
Zn
match
Ga
match
Ge
match
As
match
Se
match
Br
match
Kr
match
Rb
match
Sr
match
Y
match
Zr
match
Nb
match
Mo
match
Tc
match
Ru
match
Rh
match
Pd
match
Ag
match
Cd
match
In
match
Sn
match
A
no match
At
no match
Ab
no match
Ad
no match
Ba
no match
Ce
no match
D
no match
E
no match
En
no match
Fa
no match
Fu
no match
Gu
no match
Ha
no match
Hu
no match
I
no match
Is
no match
J
no match
Ka
no match
Kb
no match
L
no match
La
no match
M
no match
Ma
no match
Ob
no match
Op
no match
Pa
no match
Pb
no match
Pe
no match
Pg
no match
Po
no match
Q
no match
Qs
no match
R
no match
Rs
no match
Sb
no match
St
no match
Ta
no match
Te
no match
U
no match
Ub
no match
Up
no match
Vw
no match
W
no match
Wv
no match
X
no match
Xe
no match
Xp
no match
Ya
no match
Yg
no match
Z
no match
Za
no match
Congratulations, your regex passes all the test cases! Remember to share this challenge.