227. trim_matches — Strip the Same Char Off Both Ends, However Many There Are

trim() only knows about whitespace, and strip_prefix peels off one occurrence. When you need to shave every leading and trailing . (or 0, or quote) off a string, reach for trim_matches.

The hand-rolled trim

You’ve got a string padded with some character and want it gone from both ends — but only the ends:

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let s = "***heading***";

// strip_prefix only removes one, and only the front
let once = s.strip_prefix('*').unwrap_or(s);
assert_eq!(once, "**heading***");

Looping strip_prefix/strip_suffix until they stop matching works, but it’s a chore. trim_matches does exactly that for you — it removes all consecutive matches from both ends and leaves the middle alone:

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let s = "***heading***";
assert_eq!(s.trim_matches('*'), "heading");

// only the ends — interior matches stay put
assert_eq!("0x00ff00".trim_matches('0'), "x00ff");

One end at a time

There are directional versions when you only care about one side:

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assert_eq!("--verbose".trim_start_matches("--"), "verbose");
assert_eq!("file.txt.bak".trim_end_matches(".bak"), "file.txt");

The pattern can be a closure or a set of chars

The argument is a Pattern, so you’re not limited to a single char. Pass a closure to trim by predicate, or an array of chars to trim any of them:

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// strip leading digits
assert_eq!("12abc34".trim_start_matches(|c: char| c.is_numeric()), "abc34");

// trim any of several characters
assert_eq!("(value)".trim_matches(['(', ')']), "value");

Note trim_end_matches("--") strips the whole substring repeatedly, not a set of chars — that’s the difference between passing "--" and passing ['-'].