diff --git a/Package.swift b/Package.swift
index f185f76..01e2267 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.2
+// swift-tools-version:4.2
 
 import PackageDescription
 
diff --git a/Sources/SemanticVersion/SemanticVersion.swift b/Sources/SemanticVersion/SemanticVersion.swift
index 9b2289e..f53daae 100644
--- a/Sources/SemanticVersion/SemanticVersion.swift
+++ b/Sources/SemanticVersion/SemanticVersion.swift
@@ -39,7 +39,7 @@ extension SemanticVersion: LosslessStringConvertible {
 
 
 extension SemanticVersion: Comparable {
-    public static func < (lhs: Self, rhs: Self) -> Bool {
+    public static func < (lhs: SemanticVersion, rhs: SemanticVersion) -> Bool {
         if lhs.major != rhs.major { return lhs.major < rhs.major }
         if lhs.minor != rhs.minor { return lhs.minor < rhs.minor }
         if lhs.patch != rhs.patch { return lhs.patch < rhs.patch }
@@ -57,17 +57,19 @@ extension SemanticVersion: Comparable {
 
 
 extension SemanticVersion {
-    public var isStable: Bool { preRelease.isEmpty && build.isEmpty }
-    public var isPreRelease: Bool { !isStable }
-    public var isMajorRelease: Bool { isStable && (major > 0 && minor == 0 && patch == 0) }
-    public var isMinorRelease: Bool { isStable && (minor > 0 && patch == 0) }
-    public var isPatchRelease: Bool { isStable && patch > 0 }
-    public var isInitialRelease: Bool { self == .init(0, 0, 0) }
+    public var isStable: Bool { return preRelease.isEmpty && build.isEmpty }
+    public var isPreRelease: Bool { return !isStable }
+    public var isMajorRelease: Bool { return isStable && (major > 0 && minor == 0 && patch == 0) }
+    public var isMinorRelease: Bool { return isStable && (minor > 0 && patch == 0) }
+    public var isPatchRelease: Bool { return isStable && patch > 0 }
+    public var isInitialRelease: Bool { return self == .init(0, 0, 0) }
 }
 
 
 // Source: https://regex101.com/r/Ly7O1x/3/
 // Linked from https://semver.org
+#if swift(>=5)
+
 let semVerRegex = NSRegularExpression(#"""
 ^
 v?                              # SPI extension: allow leading 'v'
@@ -91,3 +93,31 @@ v?                              # SPI extension: allow leading 'v'
 )?
 $
 """#, options: [.allowCommentsAndWhitespace])
+
+#else
+
+let semVerRegex = NSRegularExpression("""
+^
+v?                              # SPI extension: allow leading 'v'
+(?<major>0|[1-9]\\d*)
+\\.
+(?<minor>0|[1-9]\\d*)
+\\.
+(?<patch>0|[1-9]\\d*)
+(?:-
+  (?<prerelease>
+    (?:0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*)
+    (?:\\.
+      (?:0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*)
+    )
+  *)
+)?
+(?:\\+
+  (?<buildmetadata>[0-9a-zA-Z-]+
+    (?:\\.[0-9a-zA-Z-]+)
+  *)
+)?
+$
+""", options: [.allowCommentsAndWhitespace])
+
+#endif
diff --git a/Tests/LinuxMain.swift b/Tests/LinuxMain.swift
new file mode 100644
index 0000000..4a04bfd
--- /dev/null
+++ b/Tests/LinuxMain.swift
@@ -0,0 +1,28 @@
+#if swift(>=5)
+
+#else
+
+import XCTest
+
+@testable import SemanticVersionTests
+
+extension SemanticVersionTests {
+    static var allTests: [(String, (SemanticVersionTests) -> () throws -> Void)] = [
+        ("test_semVerRegex_valid", test_semVerRegex_valid),
+        ("test_allow_leading_v", test_allow_leading_v),
+        ("test_semVerRegex_invalid", test_semVerRegex_invalid),
+        ("test_init", test_init),
+        ("test_description", test_description),
+        ("test_Comparable", test_Comparable),
+        ("test_isStable", test_isStable),
+        ("test_isMajorRelease", test_isMajorRelease),
+        ("test_isMinorRelease", test_isMinorRelease),
+        ("test_isPatchRelease", test_isPatchRelease),
+    ]
+}
+
+XCTMain([
+    testCase(SemanticVersionTests.allTests)
+])
+
+#endif