See issue #995

shell.sed() can accept either a JavaScript RegExp object or a String as its search_regex parameter. We should consider the following behavior:

• If it's a JavaScript RegExp, treat this as the pattern to use (don't try to reinterpret this value, just use it for matching).
• If it's a String, treat this as if it's an unextended regex (which is how unix sed operates by default). Ex. ( and ) should match literal parentheses, and not denote a capture group.

The current behavior is to treat Strings like extended regular expressions.