You can get quite a long way just using the Elixir lexer. Especially now that in Elixir 1.16 its lexing errors are better for user experience.
Example
iex> Code.string_to_quoted "(\"Modesty Blase\" or \"Willie Garvin\")"
{:ok, {:or, [line: 1], ["Modesty Blase", "Willie Garvin"]}}
iex> Code.string_to_quoted "a or (b and c)"
{:ok,
{:or, [line: 1],
[
{:a, [line: 1], nil},
{:and, [line: 1], [{:b, [line: 1], nil}, {:c, [line: 1], nil}]}
]}}
iex> Code.string_to_quoted "a or (b and c"
{:error,
{[opening_delimiter: :"(", line: 1, column: 14],
"missing terminator: ) (for \"(\" starting at line 1)", ""}}
You would then need a parser to ensure you have a valid expression for your query language but I think thats really quite easy compared to the lexical part given your reasonably straight forward requirements.
