Multiple ignored CSS blocks

If there are multiple ignored CSS blocks within a single clean-css
warning message, only the first is replaced with the ignore code.
This means that some ingored blocks are simply deleted from the output.

The problem is simply that the RegExp is only applied once, despite
having the global flag set on it.  This is fixed by calling `exec` on
the RegExp until it yields no further matches.

Added a test for this error case.

Fixes #180

src/htmlminifier.js

@@ -904,14 +904,37 @@ async function minifyHTML(value, options, partialMarkup) {
               });
 
               const ids = [];
-              new CleanCSS().minify(wrapCSS(text, type)).warnings.forEach(function (warning) {
-                const match = uidPattern.exec(warning);
-                if (match) {
-                  const id = uidAttr + match[2] + uidAttr;
-                  text = text.replace(id, ignoreCSS(id));
-                  ids.push(id);
+              // Loop removing a single ID at a time from the warnings, a
+              // warning might contain multiple IDs in the context, but we only
+              // handle the first match on each attempt.
+              while (true) {
+                const minifyTest = new CleanCSS().minify(wrapCSS(text, type));
+                if (minifyTest.warnings.length === 0) {
+                  // There are no warnings.
+                  break;
                 }
-              });
+                minifyTest.warnings.forEach(function (warning) {
+                  // It is very important to reset the RegExp before searching
+                  // as it's re-used each time.
+                  uidPattern.lastIndex = 0;
+                  const match = uidPattern.exec(warning);
+                  if (match) {
+                    const id = uidAttr + match[2] + uidAttr;
+                    // Only substitute each ID once, if this has come up
+                    // multiple times, then we need to abort.
+                    if (ids.indexOf(id) !== -1) {
+                      uidPattern.lastIndex = 0;
+                    } else {
+                      text = text.replace(id, ignoreCSS(id));
+                      ids.push(id);
+                    }
+                  }
+                });
+                if (uidPattern.lastIndex === 0) {
+                  // There was a warning that didn't match the pattern.
+                  break;
+                }
+              }
 
               return fn(text, type).then(chunk => {
                 ids.forEach(function (id) {

tests/minifier.spec.js

@@ -3594,3 +3594,14 @@ test('minify Content-Security-Policy', async () => {
   input = '<meta http-equiv="content-security-policy" content="default-src \'self\'; img-src https://*;">';
   expect(await minify(input)).toBe(input);
 });
+
+test('minify CSS multiple ignore in single warning', async () => {
+  const input = '<style type="text/css">\n{% ignore1 %}\na {\n{% ignore2 %}\n}\n{% ignore3 %}\n</style>';
+  const output = '<style type="text/css">\n{% ignore1 %}\na{\n{% ignore2 %}\n}\n{% ignore3 %}\n</style>';
+  expect(await minify(input, {
+    ignoreCustomFragments: [/\{%[\s\S]*?%\}/],
+    minifyCSS: {
+      level: 0
+    }
+  })).toBe(output);
+});