fix(config): remove trailing newlines in regexes (#373)

Closes #372

Fun YAML fact of the day:

What is the difference between how these two expressions are parsed?

```yaml
foo: >
  bar
```

```yaml
foo: >-
  bar
```

They are invisible in yaml, but when you evaluate them to JSON the
difference is obvious:

```json
{
  "foo": "bar\n"
}
```

```json
{
  "foo": "bar"
}
```

User-Agent strings, URL path values, and HTTP headers _do_ end in
newlines in HTTP/1.1 wire form, but that newline is usually stripped
before the server actually handles it. Also HTTP/2 is a thing and does
not terminate header values with newlines.

This change makes Anubis more aggressively detect mistaken uses of the
yaml `>` operator and nudges the user into using the yaml `>-` operator
which does not append the trailing newline.

I had honestly forgotten about this YAML behavior because it wasn't
relevant for so long. Oops! Glad I released a beta.

Whenever you get into this state, Anubis will throw a config parsing
error and then give you a message hinting at the folly of your ways.

```
config.Bot: regular expression ends with newline (try >- instead of > in yaml)
```

Big thanks to https://yaml-multiline.info, this helped me realize my
folly instantly.

@aiverson, this is official permission to say "told you so".

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/policy/config/config.go

@@ -24,6 +24,7 @@ var (
 	ErrInvalidPathRegex                  = errors.New("config.Bot: invalid path regex")
 	ErrInvalidHeadersRegex               = errors.New("config.Bot: invalid headers regex")
 	ErrInvalidCIDR                       = errors.New("config.Bot: invalid CIDR")
+	ErrRegexEndsWithNewline              = errors.New("config.Bot: regular expression ends with newline (try >- instead of > in yaml)")
 	ErrInvalidImportStatement            = errors.New("config.ImportStatement: invalid source file")
 	ErrCantSetBotAndImportValuesAtOnce   = errors.New("config.BotOrImport: can't set bot rules and import values at the same time")
 	ErrMustSetBotOrImportRules           = errors.New("config.BotOrImport: rule definition is invalid, you must set either bot rules or an import statement, not both")
@@ -91,12 +92,20 @@ func (b BotConfig) Valid() error {
 	}
 
 	if b.UserAgentRegex != nil {
+		if strings.HasSuffix(*b.UserAgentRegex, "\n") {
+			errs = append(errs, fmt.Errorf("%w: user agent regex: %q", ErrRegexEndsWithNewline, *b.UserAgentRegex))
+		}
+
 		if _, err := regexp.Compile(*b.UserAgentRegex); err != nil {
 			errs = append(errs, ErrInvalidUserAgentRegex, err)
 		}
 	}
 
 	if b.PathRegex != nil {
+		if strings.HasSuffix(*b.PathRegex, "\n") {
+			errs = append(errs, fmt.Errorf("%w: path regex: %q", ErrRegexEndsWithNewline, *b.PathRegex))
+		}
+
 		if _, err := regexp.Compile(*b.PathRegex); err != nil {
 			errs = append(errs, ErrInvalidPathRegex, err)
 		}
@@ -108,6 +117,10 @@ func (b BotConfig) Valid() error {
 				continue
 			}
 
+			if strings.HasSuffix(expr, "\n") {
+				errs = append(errs, fmt.Errorf("%w: header %s regex: %q", ErrRegexEndsWithNewline, name, expr))
+			}
+
 			if _, err := regexp.Compile(expr); err != nil {
 				errs = append(errs, ErrInvalidHeadersRegex, err)
 			}