diff --git a/humanfriendly/tests.py b/humanfriendly/tests.py
index 72dad99..fc7af22 100644
--- a/humanfriendly/tests.py
+++ b/humanfriendly/tests.py
@@ -111,6 +111,7 @@
     concatenate,
     dedent,
     generate_slug,
+    join_lines,
     pluralize,
     random_string,
     trim_empty_lines,
@@ -130,6 +131,21 @@ class HumanFriendlyTestCase(TestCase):
 
     """Container for the `humanfriendly` test suite."""
 
+    def test_join_lines_consecutive_short_lines(self):
+        """Join every hard wrap even when an intervening line has one character."""
+        assert join_lines('a\nb\nc\nd') == 'a b c d'
+        assert join_lines('one\na\nword') == 'one a word'
+
+    def test_join_lines_windows_line_endings(self):
+        """Join CRLF hard wraps without leaving carriage returns behind."""
+        assert join_lines('first\r\nsecond\r\nthird') == 'first second third'
+        assert join_lines('a\r\nb\r\nc') == 'a b c'
+
+    def test_join_lines_preserves_paragraphs_and_indentation(self):
+        """Leave paragraph boundaries and indented lines intact."""
+        for text in ('a\n\nb', 'a\r\n\r\nb', 'a\n b', 'a \nb', '\na\n', ''):
+            assert join_lines(text) == text
+
     def test_case_insensitive_dict(self):
         """Test the CaseInsensitiveDict class."""
         # Test support for the dict(iterable) signature.
diff --git a/humanfriendly/text.py b/humanfriendly/text.py
index a257a6a..7b30e08 100644
--- a/humanfriendly/text.py
+++ b/humanfriendly/text.py
@@ -289,7 +289,7 @@ def join_lines(text):
     break :func:`join_lines()` (in that case you can use :func:`dedent()`
     before calling :func:`join_lines()`).
     """
-    return re.sub(r'(\S)\n(\S)', r'\1 \2', text)
+    return re.sub(r'(?<=\S)\r?\n(?=\S)', ' ', text)
 
 
 def pluralize(count, singular, plural=None):