diff --git a/src/prettify.js b/src/prettify.js
index 576611f1..dba5493e 100644
--- a/src/prettify.js
+++ b/src/prettify.js
@@ -239,6 +239,51 @@ function PR_endsWith(s, suffix) {
          suffix == s.substring(s.length - suffix.length, s.length);
 }
 
+/** a set of tokens that can precede a regular expression literal in javascript.
+  * http://www.mozilla.org/js/language/js20/rationale/syntax.html has the full
+  * list, but I've removed ones that might be problematic when seen in languages
+  * that don't support regular expression literals.
+  *
+  * <p>Specifically, I've removed any keywords that can't precede a regexp
+  * literal in a syntactically legal javascript program, and I've removed the
+  * "in" keyword since it's not a keyword in many languages, and might be used
+  * as a count of inches.
+  * @private
+  */
+var REGEXP_PRECEDER_PATTERN = (function () {
+    var preceders = [
+        "!", "!=", "!==", "#", "%", "%=", "&", "&&", "&&=",
+        "&=", "(", "*", "*=", /* "+", */ "+=", ",", /* "-", */ "-=",
+        "->", /*".", "..", "...", handled below */ "/", "/=", ":", "::", ";",
+        "<", "<<", "<<=", "<=", "=", "==", "===", ">",
+        ">=", ">>", ">>=", ">>>", ">>>=", "?", "@", "[",
+        "^", "^=", "^^", "^^=", "{", "|", "|=", "||",
+        "||=", "~", "break", "case", "continue", "delete",
+        "do", "else", "finally", "instanceof",
+        "return", "throw", "try", "typeof"
+        ];
+    var pattern = '(?:' +
+      '(?:(?:^|[^0-9\.])\\.{1,3})|' +  // a dot that's not part of a number
+      '(?:(?:^|[^\\+])\\+)|' +  // allow + but not ++
+      '(?:(?:^|[^\\-])-)'  // allow - but not --
+      ;
+    for (var i = 0; i < preceders.length; ++i) {
+      var preceder = preceders[i];
+      if (PR_isWordChar(preceder.charAt(0))) {
+        pattern += '|\\b' + preceder;
+      } else {
+        pattern += '|' + preceder.replace(/([^=<>:&])/g, '\\$1');
+      }
+    }
+    pattern += ')\\s*$';  // matches at end
+    return new RegExp(pattern);
+    // CAVEAT: this does not properly handle the case where a regular expression
+    // immediately follows another since a regular expression may have flags
+    // for case-sensitivity and the like.  Having regexp tokens adjacent is not
+    // valid in any language I'm aware of, so I'm punting.
+    // TODO: maybe style special characters inside a regexp as punctuation.
+  })();
+
 /** true iff prefix matches the first prefix characters in chars[0:len].
   * @private
   */
@@ -679,6 +724,8 @@ function PR_splitStringAndCommentTokens(chunks) {
   var state = 0;  // FSM state variable
   var delim = -1;  // string delimiter
   var k = 0;  // absolute position of beginning of current chunk
+  var lookBehind = [];  // the last 16 characters processed collapsing space
+  var lastCh = '';
 
   for (var ci = 0, nc = chunks.length; ci < nc; ++ci) {
     var chunk = chunks[ci];
@@ -699,8 +746,8 @@ function PR_splitStringAndCommentTokens(chunks) {
           } else if (ch == '/') {
             state = 3;
           } else if (ch == '#') {
-            tokenEnds.push(new PR_TokenEnd(k + i, PR_PLAIN));
             state = 4;
+            tokenEnds.push(new PR_TokenEnd(k + i, PR_PLAIN));
           }
         } else if (1 == state) {
           if (ch == delim) {
@@ -719,10 +766,21 @@ function PR_splitStringAndCommentTokens(chunks) {
             state = 5;
             tokenEnds.push(new PR_TokenEnd(k + last, PR_PLAIN));
           } else {
-            state = 0;
-            // next loop will reenter state 0 without same value of i, so
-            // ch will be reconsidered as start of new token.
-            next = i;
+            // check the last token and see if we should treat this as the start
+            // of a regular expression literal.
+            if ((!lookBehind.length ||
+                 REGEXP_PRECEDER_PATTERN.test(lookBehind.join('')))) {
+              // treat regular expression as a string with delimiter /
+              state = 1;
+              delim = '/';
+              tokenEnds.push(new PR_TokenEnd(k + last, PR_PLAIN));
+            } else {
+              state = 0;
+              // next loop will reenter state 0 without same value of i, so
+              // ch will be reconsidered as start of new token.
+              next = i;
+              continue;
+            }
           }
         } else if (4 == state) {
           if (ch == '\r' || ch == '\n') {
@@ -737,10 +795,23 @@ function PR_splitStringAndCommentTokens(chunks) {
           if (ch == '/') {
             state = 0;
             tokenEnds.push(new PR_TokenEnd(k + next, PR_COMMENT));
+            continue;  // skip lookbehind
           } else if (ch != '*') {
             state = 5;
           }
         }
+
+        // push char on lookbehind if it's not a comment token.  Don't
+        // waste space with lots of space ; just leave enough to indicate
+        // boundaries.
+        if (3 > state || state > 6) {
+          var isSpace = PR_isSpaceChar(ch);
+          if (!(lastCh === ' ' && isSpace)) {
+            if (lookBehind.length > 16) { lookBehind.shift(); }
+            lastCh = isSpace ? ' ' : ch;
+            lookBehind.push(lastCh);
+          }
+        }
       }
     }
     k += s.length;
diff --git a/tests/prettify_test.html b/tests/prettify_test.html
index 93d9fa0c..99eaba29 100644
--- a/tests/prettify_test.html
+++ b/tests/prettify_test.html
@@ -150,6 +150,47 @@ <h1>Javascript</h1>
 document.write(fib(10));
 </pre>
 
+<h1>Issue 12 - Javascript Regular Expressions</h1>
+<pre class="prettyprint" id="issue12">
+/foo/;  // a slash starting a line treated as a regexp beginning
+"foo".match(/fo+$/);
+// this line comment not treated as a regular expressions
+"foo /bar/".test(/"baz"/);  // test string and regexp boundaries
+var division = /\b\d+\/\d+/g;  // test char sets and escaping of specials
+var allSpecials = /([^\(\)\[\]\{\}\-\?\+\*\.\^\$\/]+)\\/;
+
+// test that slash used in numeric context treated as an operator
+1 / 2;
+1. / x;
+x / y;
+(x) / y;
+1 /* foo */ / 2;
+1 /* foo *// 2;
+1/2;
+1./x;
+x/y;
+(x)/y;
+
+// test split over two lines.  line comment should not fool it
+1//
+/2;
+
+x++/y;
+x--/y;
+x[y] / z;
+f() / n;
+
+// test that slash after non postfix operator is start of regexp
+log('matches = ' + /foo/.test(foo));
+
+// test keyword preceders
+return /a regexp/;
+division = notreturn / not_a_regexp / 2;  // keyword suffix does not match
+
+// & not used as prefix operator in javascript but this should still work
+&/foo/;
+</pre>
+
 <h1>Perl</h1>
 <pre class="prettyprint" id="perl">
 #!/usr/bin/perl
@@ -747,7 +788,7 @@ <h1>Bug 8 - tabs mangled</h1>
       '&nbsp; &nbsp; &nbsp; `END`COM// PHP has a plethora of comment types' +
           '`END`PLN<br>' +
       '&nbsp; &nbsp; &nbsp; `END`COM\/* What is a<br>' +
-      '&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;"plethora"? */`END`PLN<br>' +
+      '&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;"plethora"? *\/`END`PLN<br>' +
       '&nbsp; &nbsp; &nbsp; `END`KWDfunction`END`PLN fib`END`PUN(`END' +
           '`PLN$n`END`PUN)`END`PLN `END`PUN{`END`PLN<br>' +
       '&nbsp; &nbsp; &nbsp; &nbsp; `END`COM# I don\'t know.`END`PLN<br>' +
@@ -827,7 +868,66 @@ <h1>Bug 8 - tabs mangled</h1>
       '`END<b>`PLNeleven`END</b>`PLN &nbsp;`END`TYPTwelve`END`PLN &nbsp;`END' +
           '<b>`PLNthirteen`END</b>`PLN &nbsp; &nbsp; &nbsp; &nbsp;`END' +
           '`TYPFourteen`END`PLN &nbsp; &nbsp; &nbsp; &nbsp;fifteen `END`' +
-          'PUN|`END')
+          'PUN|`END'),
+  issue12: (
+      '`STR/foo/`END`PUN;`END`PLN &nbsp;`END`COM// a slash starting a line ' +
+          'treated as a regexp beginning`END`PLN<br>' +
+      '`END`STR"foo"`END`PUN.`END`PLNmatch`END`PUN(`END`STR/fo+$/`END' +
+          '`PUN);`END`PLN<br>' +
+      '`END`COM// this line comment not treated as a regular expressions`END' +
+          '`PLN<br>' +
+      '`END`STR"foo /bar/"`END`PUN.`END`PLNtest`END`PUN(`END`STR/"baz"/`END' +
+          '`PUN);`END`PLN &nbsp;`END`COM// test string and regexp boundaries' +
+          '`END`PLN<br>' +
+      '`END`KWDvar`END`PLN division `END`PUN=`END`PLN `END' +
+          '`STR/\\b\\d+\\/\\d+/`END`PLNg`END`PUN;`END`PLN &nbsp;`END' +
+          '`COM// test char sets and escaping of specials`END`PLN<br>' +
+      '`END`KWDvar`END`PLN allSpecials `END`PUN=`END`PLN `END' +
+          '`STR/([^\\(\\)\\[\\]\\{\\}\\-\\?\\+\\*\\.\\^\\$\\/]+)\\\\/`END' +
+          '`PUN;`END`PLN<br>' +
+      '<br>' +
+      '`END`COM// test that slash used in numeric context treated as an ' +
+          'operator`END`PLN<br>' +
+      '`END`LIT1`END`PLN `END`PUN/`END`PLN `END`LIT2`END`PUN;`END`PLN<br>' +
+      '`END`LIT1`END`PUN.`END`PLN `END`PUN/`END`PLN x`END`PUN;`END`PLN<br>' +
+      'x `END`PUN/`END`PLN y`END`PUN;`END`PLN<br>' +
+      '`END`PUN(`END`PLNx`END`PUN)`END`PLN `END`PUN/`END`PLN y`END`PUN;`END' +
+          '`PLN<br>' +
+      '`END`LIT1`END`PLN `END`COM/* foo */`END`PLN `END`PUN/`END`PLN `END' +
+          '`LIT2`END`PUN;`END`PLN<br>' +
+      '`END`LIT1`END`PLN `END`COM/* foo */`END`PUN/`END`PLN `END`LIT2`END' +
+          '`PUN;`END`PLN<br>' +
+      '`END`LIT1`END`PUN/`END`LIT2`END`PUN;`END`PLN<br>' +
+      '`END`LIT1`END`PUN./`END`PLNx`END`PUN;`END`PLN<br>' +
+      'x`END`PUN/`END`PLNy`END`PUN;`END`PLN<br>' +
+      '`END`PUN(`END`PLNx`END`PUN)/`END`PLNy`END`PUN;`END`PLN<br>' +
+      '<br>' +
+      '`END`COM// test split over two lines. &nbsp;line comment should not ' +
+          'fool it`END`PLN<br>' +
+      '`END`LIT1`END`COM//`END`PLN<br>' +
+      '`END`PUN/`END`LIT2`END`PUN;`END`PLN<br>' +
+      '<br>' +
+      'x`END`PUN++/`END`PLNy`END`PUN;`END`PLN<br>' +
+      'x`END`PUN--/`END`PLNy`END`PUN;`END`PLN<br>' +
+      'x`END`PUN[`END`PLNy`END`PUN]`END`PLN `END`PUN/`END`PLN z`END`PUN;`END' +
+          '`PLN<br>' +
+      'f`END`PUN()`END`PLN `END`PUN/`END`PLN n`END`PUN;`END`PLN<br>' +
+      '<br>' +
+      '`END`COM// test that slash after non postfix operator is start of ' +
+          'regexp`END`PLN<br>' +
+      'log`END`PUN(`END`STR\'matches = \'`END`PLN `END`PUN+`END`PLN `END' +
+          '`STR/foo/`END`PUN.`END`PLNtest`END`PUN(`END`PLNfoo`END`PUN));`END' +
+          '`PLN<br>' +
+      '<br>' +
+      '`END`COM// test keyword preceders`END`PLN<br>' +
+      '`END`KWDreturn`END`PLN `END`STR/a regexp/`END`PUN;`END`PLN<br>' +
+      'division `END`PUN=`END`PLN notreturn `END`PUN/`END`PLN not_a_regexp ' +
+          '`END`PUN/`END`PLN `END`LIT2`END`PUN;`END`PLN &nbsp;`END`COM// ' +
+          'keyword suffix does not match`END`PLN<br>' +
+      '<br>' +
+      '`END`COM// &amp; not used as prefix operator in javascript but this ' +
+          'should still work`END`PLN<br>' +
+      '`END`PUN&amp;`END`STR/foo/`END`PUN;`END')
 };