From a945c035ce9e3d5fda81a05f95f3ea1788a19ece Mon Sep 17 00:00:00 2001
From: Gabriel Schneider <gbrls@0x4200.cafe>
Date: Tue, 29 Sep 2026 13:43:52 -0300
Subject: A ^ after | anchors its branch: ^def|^  finds lines starting either
 way

mvzr takes ^ only at its pattern's start, so in ^def|^  the second branch never matched mid-text. An alternation whose every top-level branch starts with ^ is now compiled as ^(def| ); a mix of anchored and unanchored branches is refused rather than silently wrong.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/regexp.zig | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

(limited to 'src')

diff --git a/src/regexp.zig b/src/regexp.zig
index 27b3c0ba..ca396ff1 100644
--- a/src/regexp.zig
+++ b/src/regexp.zig
@@ -48,6 +48,11 @@ pub const Regex = struct {
     /// ends of the whole text and so never match where sam would.
     pub fn compile(pat: []const u8) error{ Bad, Anchor }!Regex {
         if (pat.len == 0) return error.Bad;
+        // mvzr takes `^` only at its pattern's start, so `^def|^ ` (a `^`
+        // after a `|`) is written `^(def| )` for it: the same lines. A mix,
+        // `^a|b`, has no such spelling and is refused rather than wrong.
+        var joined: [258]u8 = undefined;
+        if (try anchoredAlternation(pat, &joined)) |whole| return compile(whole);
         var buf: [256]u8 = undefined;
         var len: usize = 0;
         var spans = false;
@@ -97,6 +102,56 @@ pub const Regex = struct {
         };
     }
 
+    /// `^a|^b` as `^(a|b)` in `out`, when the pattern is an alternation at
+    /// its top level and every branch starts with `^`; null when it is not
+    /// one, or no branch does.
+    fn anchoredAlternation(pat: []const u8, out: *[258]u8) error{Bad}!?[]const u8 {
+        var bars: [16]usize = undefined;
+        var n: usize = 0;
+        var depth: usize = 0;
+        var in_class = false;
+        var i: usize = 0;
+        while (i < pat.len) : (i += 1) {
+            const c = pat[i];
+            if (c == '\\') {
+                i += 1;
+            } else if (in_class) {
+                in_class = c != ']';
+            } else if (c == '[') {
+                in_class = true;
+            } else if (c == '(') {
+                depth += 1;
+            } else if (c == ')') {
+                depth -|= 1;
+            } else if (c == '|' and depth == 0) {
+                if (n == bars.len) return error.Bad;
+                bars[n] = i;
+                n += 1;
+            }
+        }
+        if (n == 0) return null;
+        var anchored: usize = 0;
+        var from: usize = 0;
+        for (0..n + 1) |k| {
+            const to = if (k < n) bars[k] else pat.len;
+            anchored += @intFromBool(to > from and pat[from] == '^');
+            from = to + 1;
+        }
+        if (anchored == 0) return null;
+        if (anchored != n + 1) return error.Bad;
+        var w = std.Io.Writer.fixed(out);
+        w.writeAll("^(") catch return error.Bad;
+        from = 0;
+        for (0..n + 1) |k| {
+            const to = if (k < n) bars[k] else pat.len;
+            if (k > 0) w.writeByte('|') catch return error.Bad;
+            w.writeAll(pat[from + 1 .. to]) catch return error.Bad;
+            from = to + 1;
+        }
+        w.writeByte(')') catch return error.Bad;
+        return w.buffered();
+    }
+
     pub const Match = struct { start: usize, end: usize };
 
     /// The first match that starts in `from..=last` and ends by `hi`, as
@@ -185,6 +240,16 @@ test "lines are haystacks: ^ and $ at each line, . never a newline, \\n spans li
     try std.testing.expectError(error.Bad, Regex.compile("a\\"));
 }
 
+test "a ^ after | anchors that branch: ^def|^  finds a line that starts either way, and a mix is refused" {
+    const text = "x def\n a\ndef b\n";
+    var both = try Regex.compile("^def|^ ");
+    try std.testing.expectEqual(@as(usize, 6), (try both.find(text, 0, text.len, text.len)).?.start);
+    try std.testing.expectEqual(@as(usize, 9), (try both.find(text, 7, text.len, text.len)).?.start);
+    try std.testing.expectError(error.Bad, Regex.compile("^def|x"));
+    var plain = try Regex.compile("a|b");
+    try std.testing.expectEqual(@as(usize, 7), (try plain.find(text, 0, text.len, text.len)).?.start);
+}
+
 test "a quoted backslash before n is no newline, and a class ends where mvzr ends it" {
     // `\\n` is a backslash then an n: the pattern stays on one line, so its
     // `.` is not made [^\n] and does match within the line.
-- 
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