How do I "interpret" escaped characters in a string?

Question

I want to process a string containing a backslash followed by an escapable character as if they were one character.

let raw = r#"\""#;
let cooked = raw.process_escape_character_magic();

Right now, raw has 2 characters: \ and ". But what I actually want is cooked, which only has one character: ".

How do I get cooked?

I was thinking about using regex, but I feel like there should probably be a better way.

Should every backslash + some character in the string be replaced with that character, or do different escape sequences mean different things? (Or, if you’d like to answer something that addresses a lot of potential questions at once: where did you get the string/which format is it in?) — Ry-
– Ry- ♦, Commented Oct 25, 2019 at 2:03
@Ry- Same meaning as in Rust's own strings, i.e. "\n" gives a newline character, etc. — kkeey
– kkeey, Commented Oct 25, 2019 at 2:21

Boiethios · Accepted Answer · 2019-10-30 10:44:48Z

4

I like using iterators in Rust, and I think that's a perfect usecase:

#[derive(Debug, PartialEq)]
enum MyError {
    EscapeAtEndOfString,
    InvalidEscapedChar(char),
}

struct InterpretEscapedString<'a> {
    s: std::str::Chars<'a>,
}

impl<'a> Iterator for InterpretEscapedString<'a> {
    type Item = Result<char, MyError>;

    fn next(&mut self) -> Option<Self::Item> {
        self.s.next().map(|c| match c {
            '\\' => match self.s.next() {
                None => Err(MyError::EscapeAtEndOfString),
                Some('n') => Ok('\n'),
                Some('\\') => Ok('\\'),
                // etc.
                Some(c) => Err(MyError::InvalidEscapedChar(c)),
            },
            c => Ok(c),
        })
    }
}

fn interpret_escaped_string(s: &str) -> Result<String, MyError> {
    (InterpretEscapedString { s: s.chars() }).collect()
}

fn main() {
    assert_eq!(interpret_escaped_string(r#""#), Ok("".into()));
    assert_eq!(interpret_escaped_string(r#"a"#), Ok("a".into()));
    assert_eq!(interpret_escaped_string(r#"\"#), Err(MyError::EscapeAtEndOfString));
    assert_eq!(interpret_escaped_string(r#"\\"#), Ok("\\".into()));
    assert_eq!(interpret_escaped_string(r#"a\n"#), Ok("a\n".into()));
    assert_eq!(interpret_escaped_string(r#"a\."#), Err(MyError::InvalidEscapedChar('.')));
}

More complete implementation of such a module in the playground.

edited Oct 30, 2019 at 10:44

answered Oct 25, 2019 at 8:43

Boiethios

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2 Comments

kkeey Over a year ago

No way to get all escaped characters? i.e. \", \`, \t, \n`, etc.? Or do we have to manually put them in the match statement? Close enough either way I suppose, but still leaves a bad taste in my mouth.

Boiethios Over a year ago

@kkeey There's no reason that would be in the std lib. That's an information internal to the compiler.

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