Ruby's scanf — the inverse of sprintf — in pure Go, MRI-compatible, no cgo.
go-ruby-scanf is a pure-Go (no cgo) reimplementation of Ruby's scanf stdlib — the deterministic, interpreter-independent core of MRI's String#scanf / IO#scanf. It parses values out of an input string according to a scanf format string, so it is the inverse of sprintf: where sprintf("%d", 42) renders "42", scanf("42", "%d") reads 42 back. It covers the full directive grammar — integers with base detection, hex/octal and MRI's quirky float forms, %s / %c, character-class sets %[…], widths, assignment suppression, and MRI's exact stopping semantics — with integers widening to *big.Int on overflow. It is bound into go-embedded-ruby by rbgo as a native module just like go-ruby-regexp and go-ruby-erb — differential-tested against MRI, 100% coverage, CI green across 6 arches.
%d / %u signed decimal, %i with base detection (0x=hex, leading 0=octal), %x / %X hex and %o octal — values widening transparently to arbitrary-precision *big.Int the moment they overflow int, just like Ruby’s Integer.
%a / %e / %f / %g and their uppercase forms, including hexadecimal floats (0x1.8p1), MRI’s 123.e+3 form, and ±Infinity saturation on overflow — matching MRI’s quirky float grammar byte-for-byte.
%s (a run of non-whitespace), %c (a single character or n with a width, which does not skip leading whitespace), and character-class sets %[…] / %[^…] with ranges, POSIX named classes (%[[:alpha:]]), and width bounds.
A decimal field width on any directive (%3d, %5s, %4f), and the assignment-suppression * flag (%*d) that consumes input but emits no value; %% matches a literal %.
Scanning stops at the first non-match, at end of input, or when the format is exhausted, returning the values converted so far — a partial match yields a shorter slice, no match an empty one. Scan runs once; ScanAll repeats the format until input is exhausted.
A wide corpus — every directive, width, set, suppression flag, literal, and partial/failed match, in both single and block forms — scanned here and by the system ruby (String#scanf) and compared #inspect-for-#inspect; 100% coverage, gofmt + go vet clean, green across all six 64-bit Go arches.
A faithful port of Ruby's scanf gem in pure Go, cgo disabled, so it cross-compiles and embeds anywhere. It scans integers (%d/%i/%x/%o), floats (%a/%e/%f/%g, including hex floats), %s / %c, character-class sets %[…], field widths, and assignment suppression — matching MRI's quirky float grammar and stopping semantics byte-for-byte. Validated differentially against the system ruby binary (String#scanf). It is a standalone, reusable module bound into the sibling org github.com/go-embedded-ruby.