Parsing

Every port has two parsers. Both reject rather than guess, and both give the same answer in every language for the same input. The normative text is PARSING.md in the hub, and every port runs its parse.tsv vectors.

Strict

Exactly year, month, day, in that order.

• Digits are ASCII 0 to 9 or Devanagari ० to ९, and may be mixed.
• Separators are -, /, ., _, \, a space, । or |. Leading, trailing and repeated separators are ignored.
• Anything else is an error, as is a fourth group or a group longer than four digits.

All of these are BS 2080-05-15:

2080/05/15
2080-5-15
  2080 / 05 / 15
/2080/05/15/
2080।05।15
२०८०/०५/१५

Lenient

For what a person types into a form. It tries the strict parser first, then:

• A month name with two numbers: the four-digit number is the year and the other is the day, in any order. 15 Shrawan 2080, Shrawan 15, 2080 and 2080 Shrawan 15 are the same date.
• Three numbers: Y M D when the first has four digits, D M Y when the last does.
• Commas count as separators. Era markers (BS, B.S., VS, वि.सं. and their variants) and the words गते and मिति are dropped.
• Month names are matched whole and without regard to case for Latin letters, from a closed list of English spellings and the Nepali names.

It refuses to guess: a two-digit year is never expanded, and input such as 05/06/07 is ambiguous, an error, rather than one of its three readings.

Errors

Both parsers report a kind your code can act on:

Kind	When
Invalid character	strict: a character that is neither a digit nor a separator
Wrong group count	strict: not exactly three numbers
Number too long	a number with more than four digits
Ambiguous	lenient: no way to tell the year from the day
Unrecognized	lenient: an unknown word, two month names, or nothing at all
Out of range, invalid month, invalid day	the numbers parse but aren’t a BS date between 1901 and 2199

Each language spells these in its own style, for example ParseErrorKind::Ambiguous in Rust or "ambiguous" in TypeScript.