Adding lookbehind assertion

spec/parser/javascript/subexp_spec.js

@@ -22,6 +22,12 @@ describe('parser/javascript/subexp.js', function() {
     '(?!test)': {
       regexp: jasmine.objectContaining({ textValue: 'test' })
     },
+    '(?<=test)': {
+      regexp: jasmine.objectContaining({ textValue: 'test' })
+    },
+    '(?<!test)': {
+      regexp: jasmine.objectContaining({ textValue: 'test' })
+    },
     '(?:test)': {
       regexp: jasmine.objectContaining({ textValue: 'test' }),
       proxy: jasmine.objectContaining({ textValue: 'test' })
@@ -110,6 +116,14 @@ describe('parser/javascript/subexp.js', function() {
         label: 'negative lookahead',
         groupCounter: 1
       },
+      '(?<=test)': {
+        label: 'positive lookbehind',
+        groupCounter: 1
+      },
+      '(?<!test)': {
+        label: 'negative lookbehind',
+        groupCounter: 1
+      },
       '(?:test)': {
         label: '',
         groupCounter: 1

src/js/parser/javascript/grammar.peg

@@ -11,7 +11,7 @@ grammar JavascriptRegexp
   repeat_spec <- ( "{" min:[0-9]+ "," max:[0-9]+ "}"
                  / "{" min:[0-9]+ ",}"
                  / "{" exact:[0-9]+ "}" ) <RepeatSpec>
-  subexp <- "(" capture:( "?<" groupname:name ">" / "?:" / "?=" / "?!" )? regexp ")" <Subexp>
+  subexp <- "(" capture:( "?<" groupname:name ">" / "?:" / "?=" / "?!" / "?<=" / "?<!" )? regexp ")" <Subexp>
   name <- alpha (alpha / numeric)*
   alpha <- [a-zA-Z]
   numeric <- [0-9]

src/js/parser/javascript/subexp.js

@@ -26,7 +26,9 @@ export default {
   labelMap: {
     '?:': '',
     '?=': 'positive lookahead',
-    '?!': 'negative lookahead'
+    '?!': 'negative lookahead',
+    '?<=': 'positive lookbehind',
+    '?<!': 'negative lookbehind'
   },
 
   // Renders the subexp into the currently set container.