diff --git a/tests/test_pattern_matcher.py b/tests/test_pattern_matcher.py
new file mode 100644
index 0000000..fc02a5f
--- /dev/null
+++ b/tests/test_pattern_matcher.py
@@ -0,0 +1,201 @@
+"""Tests for the pattern matching engine."""
+
+import re
+import pytest
+from src.pattern_matcher import (
+    PatternMatcher,
+    PatternLibrary,
+    MatchLocation,
+    LANGUAGE_EXTENSIONS,
+    EXTENSION_TO_LANGUAGE,
+)
+
+
+class TestPatternMatcher:
+    """Tests for PatternMatcher class."""
+
+    def test_simple_pattern_match(self):
+        """Test basic regex pattern matching."""
+        matcher = PatternMatcher(r"def\s+\w+")
+        content = "def hello():\n    pass\ndef world():"
+        matches = matcher.find_matches(content, "test.py")
+
+        assert len(matches) == 2
+        assert matches[0].line_number == 1
+        assert matches[1].line_number == 3
+
+    def test_invalid_pattern_raises_error(self):
+        """Test that invalid regex raises error when compiled."""
+        matcher = PatternMatcher(r"[")
+        try:
+            _ = matcher.compiled
+            pytest.fail("Should have raised an error")
+        except (ValueError, re.error):
+            pass
+
+    def test_valid_pattern_no_error(self):
+        """Test that valid regex doesn't raise error."""
+        matcher = PatternMatcher(r"[unclosed")
+        assert matcher is not None
+
+    def test_no_matches(self):
+        """Test pattern with no matches."""
+        matcher = PatternMatcher(r"notgoingtomatch")
+        content = "def hello(): pass"
+        matches = matcher.find_matches(content, "test.py")
+
+        assert len(matches) == 0
+
+    def test_matches_extension(self):
+        """Test file extension matching."""
+        matcher = PatternMatcher(r"test")
+
+        assert matcher.matches_extension("test.py") is True
+        assert matcher.matches_extension("test.js") is True
+        assert matcher.matches_extension("test.py") is True
+        assert matcher.matches_extension("test") is False
+        assert matcher.matches_extension("test.txt") is False
+
+    def test_get_language(self):
+        """Test language detection from extension."""
+        matcher = PatternMatcher(r"test")
+
+        assert matcher.get_language("file.py") == "python"
+        assert matcher.get_language("file.js") == "javascript"
+        assert matcher.get_language("file.ts") == "typescript"
+        assert matcher.get_language("file.go") == "go"
+        assert matcher.get_language("file.rs") == "rust"
+        assert matcher.get_language("file.java") == "java"
+        assert matcher.get_language("file.unknown") is None
+
+    def test_count_matches(self):
+        """Test counting matches in content."""
+        matcher = PatternMatcher(r"\b\w+\b")
+        content = "hello world test one"
+        count = matcher.count_matches(content)
+
+        assert count == 4
+
+    def test_has_matches(self):
+        """Test checking if content has matches."""
+        matcher = PatternMatcher(r"def\s+\w+")
+
+        assert matcher.has_matches("def hello():") is True
+        assert matcher.has_matches("class Hello:") is False
+
+    def test_case_sensitive_match(self):
+        """Test case-sensitive matching."""
+        matcher = PatternMatcher(r"Hello")
+        content = "Hello hello HELLO"
+
+        matches = matcher.find_matches(content, "test.txt")
+        assert len(matches) == 1
+
+    def test_multiline_pattern(self):
+        """Test patterns spanning multiple lines."""
+        PatternMatcher(r"def\s+\w+\s*\n\s*:", flags=0)
+
+    def test_special_characters_in_pattern(self):
+        """Test pattern with special regex characters."""
+        matcher = PatternMatcher(r"\d+\.\d+")
+        content = "Numbers like 3.14 and 2.71"
+        matches = matcher.find_matches(content, "test.txt")
+
+        assert len(matches) == 2
+
+
+class TestPatternLibrary:
+    """Tests for PatternLibrary class."""
+
+    def test_get_preset(self):
+        """Test getting a preset pattern."""
+        pattern = PatternLibrary.get_pattern("python-dataclass")
+        assert pattern is not None
+        assert "@dataclass" in pattern
+
+    def test_get_nonexistent_preset(self):
+        """Test getting a preset that doesn't exist."""
+        pattern = PatternLibrary.get_pattern("nonexistent")
+        assert pattern is None
+
+    def test_list_presets(self):
+        """Test listing all presets."""
+        presets = PatternLibrary.list_presets()
+
+        assert isinstance(presets, list)
+        assert len(presets) > 0
+        assert "python-dataclass" in presets
+        assert "react-useeffect" in presets
+
+    def test_get_presets_by_category(self):
+        """Test getting presets organized by category."""
+        categories = PatternLibrary.get_presets_by_category()
+
+        assert isinstance(categories, dict)
+        assert "Python" in categories
+        assert "JavaScript/TypeScript" in categories
+        assert "Go" in categories
+
+    def test_all_presets_are_valid_regex(self):
+        """Test that all presets are valid regex patterns."""
+        for name, pattern in PatternLibrary.PRESETS.items():
+            try:
+                PatternMatcher(pattern)
+            except ValueError:
+                pytest.fail(f"Preset '{name}' has invalid regex: {pattern}")
+
+
+class TestMatchLocation:
+    """Tests for MatchLocation dataclass."""
+
+    def test_match_location_creation(self):
+        """Test creating a MatchLocation."""
+        match = MatchLocation(
+            file_path="test.py",
+            line_number=10,
+            line_content="def hello():" ,
+            match_start=0,
+            match_end=11,
+        )
+
+        assert match.file_path == "test.py"
+        assert match.line_number == 10
+        assert match.line_content == "def hello():" 
+        assert match.match_start == 0
+        assert match.match_end == 11
+
+    def test_match_location_to_dict(self):
+        """Test converting MatchLocation to dictionary."""
+        match = MatchLocation(
+            file_path="test.py",
+            line_number=5,
+            line_content="pass",
+            match_start=0,
+            match_end=4,
+        )
+
+        data = match.to_dict()
+
+        assert data["file_path"] == "test.py"
+        assert data["line_number"] == 5
+        assert data["line_content"] == "pass"
+
+
+class TestLanguageExtensions:
+    """Tests for language extension mappings."""
+
+    def test_python_extensions(self):
+        """Test Python file extensions."""
+        for ext in LANGUAGE_EXTENSIONS["python"]:
+            assert EXTENSION_TO_LANGUAGE.get(ext) == "python"
+
+    def test_javascript_extensions(self):
+        """Test JavaScript file extensions."""
+        for ext in LANGUAGE_EXTENSIONS["javascript"]:
+            assert EXTENSION_TO_LANGUAGE.get(ext) == "javascript"
+
+    def test_all_extensions_mapped(self):
+        """Test that all extensions are mapped to a language."""
+        for lang, extensions in LANGUAGE_EXTENSIONS.items():
+            for ext in extensions:
+                assert ext in EXTENSION_TO_LANGUAGE, f"Extension {ext} not mapped"