The Regex Coach - Tutorial step 18

Switch to the "Step" tab, type the regular expression (?:[a]*b|a+ac)y into the regex pane, and type the string xaacy into the target pane. Click the "Start" button.

Screen Shot

By pressing the "Next" button repeatedly you can now see a step-by-step visual tour of how the regex engine tries to match the target string.

While you do this the red background in the upper step pane (the step regex pane) highlights which subexpression the engine is currently trying to match. The message area below the step regex pane (the step regex message area) shows the result of this attempt. If there's a (sub-)match the corresponding part of the target string in the lower pane (the step target pane) is also highlighted in red.

The part of the target string before this match is either yellow (which means that this part has already matched) or grey (which means that this part isn't taken into consideration).

The following table shows what will happen in this particular example.

step regex panestep regex messagestep target pane
1.(?:[a]*b|a+ac)yNo sub-match at 0xaacy
The initial state after the "Start" button has been pressed. The engine tries to match [a] against x (position 0) but doesn't succeed.
2.(?:[a]*b|a+ac)ySub-match from 0 to 0xaacy
Which means that [a]* did succeed - zero repetitions, i.e. a zero-length match from 0 to 0.
3.(?:[a]*b|a+ac)yNo sub-match at 0xaacy
Now we must match the b (still at the same position). Nope, doesn't work...
4.(?:[a]*b|a+ac)yNo sub-match at 0xaacy
Which in turn means that the whole subexpression [a]*b didn't succeed.
5.(?:[a]*b|a+ac)yNo sub-match at 0xaacy
So the engine tries the other half of the alternation - let's start with one a. Again, no match.
6.(?:[a]*b|a+ac)yNo sub-match at 0xaacy
Which implies that a+ can't match.
7.(?:[a]*b|a+ac)yNo sub-match at 0xaacy
Which in turn implies that a+ac can't match.
8.(?:[a]*b|a+ac)yNo sub-match at 0xaacy
So, finally, the whole alternation is doomed to fail at position 0.
9.(?:[a]*b|a+ac)ySub-match from 1 to 2xaacy
We start anew at position 1. The x at position 0 now has a grey background because we don't look at it anymore. Again, we start with [a] and this time we suceed. The sub-match is shown with a red background in the step target pane.
10.(?:[a]*b|a+ac)ySub-match from 2 to 3xaacy
[a] matches again at position 2. The match we have so far is marked with a yellow background.
11.(?:[a]*b|a+ac)ySub-match from 1 to 3xaacy
So, [a]* can match the sequence aa.
12.(?:[a]*b|a+ac)yNo sub-match at 3xaacy
But we can't match b from here.
13.(?:[a]*b|a+ac)ySub-match from 1 to 2xaacy
Let's see if it helps to make the [a]* match shorter. (This is the technique known as backtracking.)
14.(?:[a]*b|a+ac)yNo sub-match at 2xaacy
Hmm, no...
15.(?:[a]*b|a+ac)ySub-match from 1 to 1xaacy
Still shorter: a zero-length match for [a]*.
16.(?:[a]*b|a+ac)yNo sub-match at 1xaacy
No, didn't help either.
17.(?:[a]*b|a+ac)yNo sub-match at 1xaacy
So, the whole left side of the alternation didn't match.
18.(?:[a]*b|a+ac)ySub-match from 1 to 2xaacy
Let's try the right side. The first a matches.
19.(?:[a]*b|a+ac)ySub-match from 2 to 3xaacy
And it matches again.
20.(?:[a]*b|a+ac)ySub-match from 1 to 3xaacy
So that the repetition a+ can match the substring aa.
21.(?:[a]*b|a+ac)yNo sub-match at 3xaacy
But that has eaten up all as, so the string ac behind the repetition can't match.
22.(?:[a]*b|a+ac)ySub-match from 1 to 2xaacy
Let's make the repetition shorter - backtracking again.
23.(?:[a]*b|a+ac)ySub-match from 2 to 4xaacy
This time it works!
24.(?:[a]*b|a+ac)ySub-match from 1 to 4xaacy
Which means that the whole right side of the alternation matched.
25.(?:[a]*b|a+ac)ySub-match from 1 to 4xaacy
Which of course means that the whole alternation matched.
26.(?:[a]*b|a+ac)ySub-match from 4 to 5xaacy
Now only the y at the end remains - and succeeds.
27.(?:[a]*b|a+ac)yMatch from 1 to 5xaacy
That's it - we're through.

Note that the "Next" button is disabled after the last step. The step panes now look exactly like their "big brothers" above them.

In case you're wondering what the "verbose" checkbox is for: Uncheck it and the stepper won't report more than one failure in a row, i.e. steps 4 to 8 and 17 above wouldn't have happened.