JSMin uses a pretty crude tokenizer-like algorithm based on assumptions of what kind of characters can precede/follow others. In the PHP port I maintain I’ve made numerous fixes to handle most syntax in the wild, but I see no fix for this:

Here’s a very boiled down version of a statement created by another minifier:

if(a)/b/.test(c)?d():e();

Since (a) follows if, we know it’s a condition, and, if truthy, the statement /b/.test(c)?d():e() will be executed.

However, JSMin has no parser and therefore no concept that (a) is an if condition; it just sees an expression. Since lots of scripts in the wild have expressions like (expression) / divisor, we’ve told JSMin to see that as division. So when JSMin arrives at )/, it incorrectly assumes / is the division operator, then incorrectly treats the following / as the beginning of a RegExp literal.

Lessons: Don’t run minified code through JSMin*, and don’t use it at all if you have access to a JavaScript/Java runtime.

*Minify already knows to skip JSMin for files ending in -min.js or .min.js.