Merge pull request #12 from kitten/fix/zero-length-matches

breaking: Only consider undefined/null failed matches

src/babel/__snapshots__/plugin.test.js.snap

···
   var node = [];
   var x;
 
-  if (x = __private.exec(state, _re_expression)) {
+  if ((x = __private.exec(state, _re_expression)) != null) {
     node.push(x);
   } else {
     state.y = y1;
···
     return;
   }
 
-  if (x = __private.exec(state, str)) {
+  if ((x = __private.exec(state, str)) != null) {
     node.push(x);
   } else {
     state.y = y1;
···
   var node = [];
   var x;
 
-  if (x = __private.exec(state, _re_expression)) {
+  if ((x = __private.exec(state, _re_expression)) != null) {
     node.push(x);
   } else {
     state.y = y1;
···
     return;
   }
 
-  if (x = __private.exec(state, \\"2\\")) {
+  if ((x = __private.exec(state, \\"2\\")) != null) {
     node.push(x);
   } else {
     state.y = y1;

src/codegen.js

···
     : `${_private}.exec(${_state}, ${ast.expression.id})`;
 
   return js`
-    if (${_match} = ${expression}) {
+    if ((${_match} = ${expression}) != null) {
       ${opts.capture ? js`${_node}.push(${_match})` : ''}
     } else {
       ${opts.onAbort}

src/core.js

···
           if (pattern.test(input))
             match = input.slice(state.y, pattern.lastIndex);
         } else {
-          match = pattern.exec(input)[0] || match;
+          const x = pattern.exec(input);
+          match = x[1] == null ? x[0] : match;
         }
 
         state.y = pattern.lastIndex;

src/core.test.js

···
 describe('interpolation parsing', () => {
   const node = match('node')`
     ${/1/}
-    ${interpolation((x) => x > 1 && x)}
+    ${interpolation((x) => (x > 1 ? x : null))}
     ${/3/}
   `;