How to use a UDF inside par_sql_expr

[farzeentitina]

I have registered a UDF successfully within SessionContext and I am able to see it in the list of the registered udfs when I call ctx.udfs()

I have a sql statement that calls this UDF and I need to use in with_column.
These are the steps I am following

let sql : &str = "testudf(a)";
let expr: Expr = df.parse_sql_expr(sql);
df  = df.with_column("testCol", expr);

this doesn't work and I receive an error saying testudf is an invalid function. however when I use it like this it works

let expr: Expr = testudf.call(vec![col("\"a\"")]);
df = df.with_column("testCol",expr);

Am i missing something in the first example where I am parsing it as a sql expression?