Cannot recognize literals when using with an operator

[zhangysh1995]

Test case:

from moz_sql_parser import parse, format

# works
s = "select '123';"
print(s)
tokens = parse(s)
print(tokens)
back = format(tokens)
print(back)

s = 'SELECT `col_double_undef_signed` FROM `table_10_binary_undef` WHERE (8221 <> "back") ORDER BY `col_char(20)_key_signed`;'
print(s)
#  doesn't work, "back" is not parsed to literal
tokens = parse(s)
print(tokens)
back = format(tokens, ansi_quotes=False)
print(back)

Output:

select '123';
{'select': {'value': {'literal': '123'}}}
SELECT '123'
SELECT `col_double_undef_signed` FROM `table_10_binary_undef` WHERE (8221 <> "back") ORDER BY `col_char(20)_key_signed`;
# "back" is not a literal
{'select': {'value': 'col_double_undef_signed'}, 'from': 'table_10_binary_undef', 'where': {'neq': [8221, 'back']}, 'orderby': {'value': 'col_char(20)_key_signed'}}
SELECT `col_double_undef_signed` FROM `table_10_binary_undef` WHERE (8221 <> `back`) ORDER BY `col_char(20)_key_signed`

[zhangysh1995]

I could locate the root cause here and here.
It seems only single quote is recognized as a literal.

# STRINGS, NUMBERS, VARIABLES
sqlString = Regex(r"\'(\'\'|\\.|[^'])*\'").addParseAction(to_string)

[klahnakoski]

@zhangysh1995 yes, only the single quote can be used for literals; double quotes are for identifiers