Why regular expression .* is slower at one place and faster at other

Question

Lately I am using a lot of regular expressions in java/groovy. For testing I routinely use regex101.com. Obviously I am looking at the regular expressions performance too.

One thing I noticed that using .* properly can significantly improve the overall performance. Primarily, using .* in between, or better to say not at the end of the regular expression is performance kill.

For example, in this regular expression the required number of steps is 27:

If I change first .* to \s*, it will reduce the steps required significantly to 16:

However, if I change second .* to \s*, it does not reduce the steps any further:

I have few questions:

1. Why the above? I dont want to compare \s and .*. I know the difference. I want to know why \s and .* costs different based on their position in the complete regex. And then the characteristics of the regex which may cost different based on their position in the overall regex (or based on any other aspect other than position, if there is any).
2. Does the steps counter given in this site really gives any indication about regex performance?
3. what other simple or similar (position related) regex performance observations you have?

Answer 1

The following is output from the debugger.

The big reason for the difference in performance is that .* will consume everything until the end of the string (except the newline). The pattern will then continue, forcing the regex to backtrack (as seen in the first image).

The reason that \s and .* perform equally well at the end of the pattern is that the greedy pattern vs. consuming whitespace makes no difference if there's nothing else to match (besides WS).

If your test string didn't end in whitespace, there would be a difference in performance, much like you saw in the first pattern - the regex would be forced to backtrack.

EDIT

You can see the performance difference if you end with something besides whitespace:

Bad:

^myname.*mahesh.*hiworld

Better:

^myname.*mahesh\s*hiworld

Even better:

^myname\s*mahesh\s*hiworld