fix: reject multiple patterns when -P/--perl-regexp is used

GNU grep's PCRE backend supports only a single pattern. Supplying
multiple patterns via repeated -e flags, or a pattern string that
contains a literal newline, must exit 2 with the message:

    the -P option only supports a single pattern

Add the validation immediately after patterns are collected, before
regex-mode selection. Add a test covering:
  - two separate -e flags with -P
  - a newline-embedded pattern string with -P
  - single -e with -P still works normally

Closes #34

src/lib.rs

@@ -255,6 +255,14 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         ));
     }
 
+    // GNU grep's PCRE backend (-P) supports only a single pattern.
+    if perl_regexp && patterns.len() > 1 {
+        return Err(USimpleError::new(
+            2,
+            "the -P option only supports a single pattern".to_string(),
+        ));
+    }
+
     // Decoded options into enums
     let regex_mode = if fixed_strings {
         RegexMode::Fixed

tests/test_grep.rs

@@ -170,6 +170,34 @@ fn pcre_features() {
         .stdout_only("42\n7\n");
 }
 
+#[test]
+fn perl_regexp_rejects_multiple_patterns() {
+    // GNU grep's PCRE backend (-P) only supports a single pattern.
+    // Multiple -e patterns must produce exit 2 and the canonical error message.
+    // See: https://github.com/uutils/grep/issues/34
+
+    // Two separate -e flags.
+    let (_s, mut c) = ucmd();
+    c.args(&["-P", "-e", "foo", "-e", "bar"])
+        .pipe_in("foo\nbar\n")
+        .fails_with_code(2)
+        .stderr_contains("the -P option only supports a single pattern");
+
+    // A newline inside the pattern string is split into multiple patterns.
+    let (_s, mut c) = ucmd();
+    c.args(&["-P", "-e", "foo\nbar"])
+        .pipe_in("foo\nbar\n")
+        .fails_with_code(2)
+        .stderr_contains("the -P option only supports a single pattern");
+
+    // A single pattern with -P must still work normally.
+    let (_s, mut c) = ucmd();
+    c.args(&["-P", "-e", r"\d+"])
+        .pipe_in("abc\n42\n")
+        .succeeds()
+        .stdout_only("42\n");
+}
+
 #[test]
 fn posix_character_classes() {
     let (_s, mut c) = ucmd();