local expr = '--username=John'
local name, value = expr:match( '^%-%-(.*)=%"?(.*)%"?' )
print( "found values for '" .. expr .. "'" )
print( name .. " = " .. value )

print( "---------------------------------" )

expr = '--username="John Doe"'
name, value = expr:match( '^%-%-(.*)="?(.-)"?$' )
print( "found values for '" .. expr .. "'" )
print( name .. " = " .. value )

print( "---------------------------------" )

expr = '--username="John Doe"'
name, value = expr:match( '^%-%-(.*)="?([^"]*)"?' )
print( "found values for '" .. expr .. "'" )
print( name .. " = " .. value )