Rust Java Printf Parser

formatjs_java_printf_parser parses Java SE 17 Formatter syntax, including Android's printf-style placeholders. It has no runtime dependencies and requires Rust 1.90 or newer.

[dependencies]
formatjs_java_printf_parser = "0.1"

use formatjs_java_printf_parser::{parse, Element};

let elements = parse("Hello %2$s, %s %<s %%").unwrap();
let indices: Vec<_> = elements.iter().filter_map(|element| match element {
    Element::Specifier(specifier) => specifier.argument.map(|argument| argument.index),
    Element::Literal(_) => None,
}).collect();
assert_eq!(indices, [2, 1, 1]);

parse(&str) returns borrowed Element::Literal and Element::Specifier nodes. Each node carries UTF-8 byte offsets into the original input. A specifier retains its raw text, original flag order, explicit index, resolved one-based argument reference, width, precision, conversion kind and uppercase mode. ParseError carries a structured ErrorKind and byte span.

Explicit indices (%2$s) do not advance the ordinary counter (%s). Relative indexing (%<s) reuses the previous consuming argument. Percent (%%) and newline (%n) conversions consume nothing and do not change either counter. Date/time conversions retain suffix case separately from the t/T prefix.

The parser checks Java grammar and argument-independent format constraints. Runtime types and values remain caller concerns: %#s needs a Java Formattable, for example. Parsing does not compare source and translated argument requirements.

For Android, decode resource XML first and respect formatted="false". Parse each plural or array item independently because argument indexing restarts for every formatted string. XML markup, backslash escapes and braces remain literal text. C printf length modifiers and iOS %@ are separate dialects.

Java Formatter oracle tests cover conversion/flag combinations and argument selection. Java is a test dependency only; consumers need no JVM. Release Please and the crates release workflow register this crate.

Validate translation catalogs#

The npm and Rust CLIs can compare Java printf argument requirements:

formatjs verify "lang/*.json" --source-locale en \
  --message-syntax java-printf --structural-equality

The parser remains independent of this policy. CLI validation compares resolved argument indices and conversion requirement sets, allows reordered/repeated references, and reports syntax errors or missing/extra/incompatible arguments. See CLI verification for input and compatibility rules.