Script mode (`--script`)

When regex’s mad-libs template ($1, ${name}) can’t compute the replacement, drop in a Rhai script callback.

Quick example: version bump

cat > bump.rhai <<'RHAI'
(parse_int(captures[1]) + 1).to_string()
RHAI

echo "version 3" | recast --stdin --script bump.rhai '(\d+)' ''
# version 4

Binding	Type	Meaning
`captures`	Array	`captures[0]` is the full match, `captures[1..]` are the named/numbered groups in order
`whole`	String	Alias for `captures[0]` (`match` is a Rhai keyword)

The return value is coerced to string and used as the replacement.

if captures[1] == "old" {
    "new"
} else {
    captures[1]                 // keep as-is
}

captures[1].to_upper()

The positional REPLACEMENT argument is still required when using --script (pass ""); its value is ignored.
Scripted scans run sequentially — the Rhai engine isn’t Sync. That’s usually fine since --script runs are dominated by per-script work, not file I/O. Use plain regex mode for big trees.
Sandbox limits: 1 M operations, 1 MiB strings, 1024 array entries, expression depth 64.
Match-count guard, idempotency check, atomic apply, and recovery all still apply.