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fix: separate a trailing capital in a default env name #446

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fix: separate a trailing capital in a default env name #446

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2 changes: 2 additions & 0 deletions env.go
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Original file line number Diff line number Diff line change
Expand Up @@ -525,6 +525,8 @@ func toEnvName(input string) string {
if unicode.IsLower(peek) || unicode.IsLower(rune(input[i-1])) {
output = append(output, underscore)
}
} else if i > 0 && unicode.IsLower(rune(input[i-1])) {

@caarlos0 caarlos0 Oct 7, 2026

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Medium: Check the preceding rune, not its final UTF-8 byte

With an exported field ƵB string, UseFieldNameByDefault: true, and Environment: map[string]string{"ƵB": "set"}, this change makes parsing silently leave the field empty instead of loading "set". The generated key changes from ƵB on the base to Ƶ_B here.

The range index i is a byte offset. Ƶ is uppercase U+01B5, encoded as C6 B5; at the terminal B, input[i-1] is 0xB5. Casting that byte to a rune produces lowercase U+00B5 (µ), so the new condition incorrectly splits an uppercase run. I verified the bytes independently, checked Go's Unicode classifications, and traced the generated key through parseFieldParams and the environment lookup; no local Go build or test was run.

Decode the actual preceding rune in this added branch, for example with utf8.DecodeLastRuneInString(input[:i]), before calling unicode.IsLower. Add toEnvName("ƵB") == "ƵB" and a parsing regression with the environment map above, retaining the MyA regression.

output = append(output, underscore)
}
}
output = append(output, unicode.ToUpper(c))
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17 changes: 17 additions & 0 deletions env_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1726,6 +1726,20 @@ func TestNoEnvKey(t *testing.T) {
isEqual(t, "", cfg.bar)
}

func TestUseFieldNameTrailingCapital(t *testing.T) {
type Config struct {
MyA string
}
var cfg Config
isNoErr(t, ParseWithOptions(&cfg, Options{
UseFieldNameByDefault: true,
Environment: map[string]string{
"MY_A": "set",
},
}))
isEqual(t, "set", cfg.MyA)
}

func TestToEnv(t *testing.T) {
for in, out := range map[string]string{
"Foo": "FOO",
Expand All @@ -1739,6 +1753,9 @@ func TestToEnv(t *testing.T) {
"SSHPort": "SSH_PORT",
"_SSH___Port_": "SSH_PORT",
"_PortHTTP": "PORT_HTTP",
"MyA": "MY_A",
"FooB": "FOO_B",
"AbC": "AB_C",
} {
t.Run(in, func(t *testing.T) {
isEqual(t, out, toEnvName(in))
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