---input---
x="$"
y="$_"
z="$abc"

---tokens---
'x'           Name.Variable
'='           Operator
'"'           Literal.String.Double
'$'           Text
'"'           Literal.String.Double
'\n'          Text.Whitespace

'y'           Name.Variable
'='           Operator
'"'           Literal.String.Double
'$_'          Name.Variable
'"'           Literal.String.Double
'\n'          Text.Whitespace

'z'           Name.Variable
'='           Operator
'"'           Literal.String.Double
'$abc'        Name.Variable
'"'           Literal.String.Double
'\n'          Text.Whitespace