Expand description

Converts to and from various cases.

Command Line Utility ccase

This library was developed for the purposes of a command line utility for converting the case of strings and filenames. You can check out ccase on Github.

Rust Library

Provides a Case enum which defines a variety of cases to convert into. Strings have implemented the Casing trait, which adds methods for case conversion.

You can convert strings into a case using the to_case method.

use convert_case::{Case, Casing};

assert_eq!("Ronnie James Dio", "ronnie james dio".to_case(Case::Title));
assert_eq!("ronnieJamesDio", "Ronnie_James_dio".to_case(Case::Camel));
assert_eq!("Ronnie-James-Dio", "RONNIE_JAMES_DIO".to_case(Case::Train));

By default, to_case will split along a set of default word boundaries, that is

  • space characters ,
  • underscores _,
  • hyphens -,
  • changes in capitalization from lowercase to uppercase aA,
  • adjacent digits and letters a1, 1a, A1, 1A,
  • and acroynms AAa (as in HTTPRequest).

For more accuracy, the from_case method splits based on the word boundaries of a particular case. For example, splitting from snake case will only use underscores as word boundaries.

use convert_case::{Case, Casing};

    "2020 04 16 My Cat Cali",
    "2020-04-16 My Cat Cali",

Case conversion can detect acronyms for camel-like strings. It also ignores any leading, trailing, or duplicate delimiters.

use convert_case::{Case, Casing};

assert_eq!("io_stream", "IOStream".to_case(Case::Snake));
assert_eq!("my_json_parser", "myJSONParser".to_case(Case::Snake));

assert_eq!("weird_var_name", "__weird--var _name-".to_case(Case::Snake));

It also works non-ascii characters. However, no inferences on the language itself is made. For instance, the digraph ij in Dutch will not be capitalized, because it is represented as two distinct Unicode characters. However, æ would be capitalized. Accuracy with unicode characters is done using the unicode-segmentation crate, the sole dependency of this crate.

use convert_case::{Case, Casing};

assert_eq!("granat-äpfel", "GranatÄpfel".to_case(Case::Kebab));
assert_eq!("Перспектива 24", "ПЕРСПЕКТИВА24".to_case(Case::Title));

// The example from str::to_lowercase documentation
let odysseus = "ὈΔΥΣΣΕΎΣ";
assert_eq!("ὀδυσσεύς", odysseus.to_case(Case::Lower));

By default, characters followed by digits and vice-versa are considered word boundaries. In addition, any special ASCII characters (besides _ and -) are ignored.

use convert_case::{Case, Casing};

assert_eq!("e_5150", "E5150".to_case(Case::Snake));
assert_eq!("10,000_days", "10,000Days".to_case(Case::Snake));
assert_eq!("HELLO, WORLD!", "Hello, world!".to_case(Case::Upper));
assert_eq!("One\ntwo\nthree", "ONE\nTWO\nTHREE".to_case(Case::Title));

You can also test what case a string is in.

use convert_case::{Case, Casing};

assert!( "css-class-name".is_case(Case::Kebab));

Note on Accuracy

The Casing methods from_case and to_case do not fail. Conversion to a case will always succeed. However, the results can still be unexpected. Failure to detect any word boundaries for a particular case means the entire string will be considered a single word.

use convert_case::{Case, Casing};

// Mistakenly parsing using Case::Snake
assert_eq!("My-kebab-var", "my-kebab-var".from_case(Case::Snake).to_case(Case::Title));

// Converts using an unexpected method
assert_eq!("my_kebab_like_variable", "myKebab-like-variable".to_case(Case::Snake));

Boundary Specificity

It can be difficult to determine how to split a string into words. That is why this case provides the from_case functionality, but sometimes that isn’t enough to meet a specific use case.

Take an identifier has the word 2D, such as scale2D. No exclusive usage of from_case will be enough to solve the problem. In this case we can further specify which boundaries to split the string on. convert_case provides some patterns for achieving this specificity. We can specify what boundaries we want to split on using the Boundary enum.

use convert_case::{Boundary, Case, Casing};

// Not quite what we want

// Remove boundary from Case::Camel
        .without_boundaries(&[Boundary::DigitUpper, Boundary::DigitLower])

// Write boundaries explicitly

The Casing trait provides initial methods, but any subsequent methods that do not resolve the conversion return a StateConverter struct. It contains similar methods as Casing.

Custom Cases

Because Case is an enum, you can’t create your own variant for your use case. However the parameters for case conversion have been encapsulated into the Converter struct which can be used for specific use cases.

Suppose you wanted to format a word like camel case, where the first word is lower case and the rest are capitalized. But you want to include a delimeter like underscore. This case isn’t available as a Case variant, but you can create it by constructing the parameters of the Converter.

use convert_case::{Case, Casing, Converter, Pattern};

let conv = Converter::new()

    conv.convert("My Special Case")

Just as with the Casing trait, you can also manually set the boundaries strings are split on. You can use any of the Pattern variants available. This even includes Pattern::Sentence which isn’t used in any Case variant. You can also set no pattern at all, which will maintain the casing of each letter in the input string. You can also, of course, set any string as your delimeter.

For more details on how strings are converted, see the docs for Converter.

Random Feature

To ensure this library had zero dependencies, randomness was moved to the random feature, which requires the rand crate. You can enable this feature by including the following in your Cargo.toml.

convert_case = { version = "^0.3.0", features = ["random"] }

This will add two additional cases: Random and PseudoRandom. You can read about their construction in the Case enum.


The parameters for performing a case conversion.

Holds information about parsing before converting into a case.


A boundary defines how a string is split into words. Some boundaries, Hyphen, Underscore, and Space, consume the character they split on, whereas the other boundaries do not.

Defines the type of casing a string can be.

A pattern is how a set of words is mutated before joining with a delimeter.


Describes items that can be converted into a case. This trait is used in conjunction with the StateConverter struct which is returned from a couple methods on Casing.