Dialects

There is no single .env standard. adotenv makes the reading rules explicit.

Presets

Presets choose parsing rules only. None enables interpolation, inheritance or file loading. Compose bare-key roles and lookup remain the caller’s job.

Portable rules

• Keys use ASCII letters, digits, _, . and -. A non-POSIX key is accepted with a warning. Other characters are invalid.
• An optional export prefix and spaces or tabs around = are accepted.
• # starts a comment at the beginning of a line or after whitespace. A=a#b has value a#b and a warning. A= # comment is assigned empty.
• Single quotes are literal. Double quotes decode \n, \r, \t, \\, \" and \$. Unknown escape pairs stay literal with a warning.
• Single and double quoted values may span lines. Original line endings remain.
• Text after a closing quote is an error unless it is a comment.
• KEY is a bare declaration with no value. KEY= and KEY="" have an empty value. Bare declarations never read the environment.
• Duplicates are retained. The last declaration wins and a warning is emitted. A last bare declaration does not fall back to an earlier value.
• A leading BOM is retained as a separate prelude. LF, CRLF and lone CR are preserved. Lone CR produces a warning.
• Backticks are literal text, not quotes.
• Parsing never expands variables or runs commands.

Custom policies

Start from a preset and use builders:

#![allow(unused)]
fn main() {
use adotenv::{BareKeys, Dialect, InlineComments};

let dialect = Dialect::NODE_DOTENV
    .with_inline_comments(InlineComments::None)
    .with_bare_keys(BareKeys::Ignore);
}

Policy groups cover:

• statement grammar and key characters;
• comments, quoting, escapes and raw values;
• quoted-key support and multiline values;
• bare declarations and duplicate lookup;
• BOM handling, line breaks and whitespace classes;
• quoted tails, malformed lines, Node object-key rules and line-byte limits.

Each document keeps its exact configuration. Document::reparse rebuilds items, entries and maps for a different dialect while reusing the source text. Snapshot IDs belong to one interpretation only.

Preset behavior

Node supports backticks, colon separators after whitespace, decoded newline normalization and its default regex fallback. __proto__ is retained as source but excluded from effective object lookup.

Python supports its named escape set, multiline single-quoted keys and Python whitespace classes. Original CRLF bytes stay in text-stream values.

Compose recognizes Unicode letter and number categories, Compose whitespace and octal escapes. Text after a quoted value can begin another statement. Its EOF bare-key behavior is retained. Dollar syntax stays literal.

Docker keeps values, hashes, quotes and internal CR bytes raw. Bare keys are retained but excluded from lookup. The scanner enforces the upstream line-byte limit and only ASCII space or tab invalidates a nonempty raw key.

Encoding

encode_value_with(value, dialect) returns one literal token or EncodeError::Unrepresentable. Success is checked in both A=<token>\n and EOF A=<token> contexts. The token does not account for arbitrary neighboring syntax or for consumers that enable interpolation.

encode_value(value) is the Portable helper. It rejects controls other than CR, LF and tab.

Portability

Document::portability(&[dialects]) reparses the retained source for each unique configuration and reports lines whose meaning differs. The document’s own dialect is always the first reading.

Each reading exposes ordered parts: declarations, blank lines, comments, invalid or ignored statements and multiline continuations. Declaration parts include key, decoded value, assignment state, export prefix, lookup participation, effective value and duplicate winner. Quote style alone is not a difference.

The check is literal. It performs no expansion, lookup, inheritance or I/O.