Fix 4.2 compile and support running tests

Sources/SemanticVersion/SemanticVersion.swift

@@ -68,6 +68,8 @@ extension SemanticVersion {
 
 // 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

Tests/LinuxMain.swift

@@ -1,8 +1,28 @@
-//
-//  File.swift
-//  
-//
-//  Created by Sven A. Schmidt on 01/09/2020.
-//
-
-import Foundation
+#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