Parsing hexadecimal characters

[mmatera]

In the task of migrating from private doctests to pytests, I hit the following bug when characters are written as escaped hexadecimal characters:

In[1]:= \.78\.79\.7A
Out[1]= xyz

but
In[2]:= F[.78.79.7A]
Syntax::sntxf: "xyz" cannot be followed by "]" (line 1 of "").
Out[2]=

and

In[3]:= ".78.79.7A"

Syntax::sntxf: "xyz" cannot be followed by "]" (line 1 of "").

However,

In[4]:= ".78.79.7A""
Out[4]= xyz

[mmatera]

Notice the comments in the test module introduced in #907