diff --git a/index.js b/index.js
index 7bcc46a..6401f87 100755
--- a/index.js
+++ b/index.js
@@ -26,7 +26,7 @@ module.exports = postcss.plugin('postcss-px-to-viewport', function (options) {
   // Not anything inside url()
   // Any digit followed by px
   // !singlequotes|!doublequotes|!url()|pixelunit
-  var pxRegex = new RegExp('"[^"]+"|\'[^\']+\'|url\\([^\\)]+\\)|(\\d*\\.?\\d+)' + opts.unitToConvert, 'ig')
+  var pxRegex = new RegExp('"[^"]+"|\'[^\']+\'|url\\([^\\)]+\\)|(\\d*\\.?\\d+)' + opts.unitToConvert, 'g')
 
   return function (css) {
 
diff --git a/spec/px-to-viewport.spec.js b/spec/px-to-viewport.spec.js
index 23632e2..628ad85 100644
--- a/spec/px-to-viewport.spec.js
+++ b/spec/px-to-viewport.spec.js
@@ -57,6 +57,17 @@ describe('value parsing', function() {
 
     expect(processed).toBe(expected);
   });
+
+  it('should not replace values with an uppercase P or X', function () {
+    var options = {
+        propList: ['*']
+    };
+    var rules = '.rule { margin: 12px calc(100% - 14PX); height: calc(100% - 20px); font-size: 12Px; line-height: 16px; }';
+    var expected = '.rule { margin: 3.75vw calc(100% - 14PX); height: calc(100% - 6.25vw); font-size: 12Px; line-height: 5vw; }';
+    var processed = postcss(pxToViewport(options)).process(rules).css;
+
+    expect(processed).toBe(expected);
+  });
 });
 
 describe('unitToConvert', function() {