(^)((\d+\.)+) Isn't there one bracket for one group, and does $3 really exist?

You're right to question how many capturing groups are created. In regular expressions, every pair of parentheses () creates a capturing group, and they're numbered by the order of their opening parenthesis, not by nesting.

Let's break it down

(^)((\d+\.)+)

(^)  Group 1: matches the start of the line (not really capturing anything useful)

((\d+\.)+)  Group 2: matches one or more digit-dot sequences like 1.2.3. or 1.

Inside that:

(\d+\.) → Group 3: matches just one digit-dot, like 1. or 2.

So yes, $3 does exist, and it represents the last match of the inner (\d+\.) pattern.

That’s why when you do a find and replace using $3, it replaces with the final \d+. part inside the repeated group. Even if it's repeated several times (like 1.2.3.), $3 will only capture the last match in this case, 3..

So in your test case:
1.2.3. Operational steps

$1 = start of line (empty string)

$2 = 1.2.3.

$3 = 3.

It's a bit messy and slow - because of non-important matches and slow because it needs to replace the found with the change to $3

We can use what we were talking about recently - when you were unsure about the difference between 

\> and \b

Here's a perfect case to find these things - and you don't need a Change To

I went one step further - but if it's not what you need we can cut it back but it's just an example and proof of concept.

^(\d\.)+(\d\.)\b

To highlight what's being found - I put in pink for show and tell

With nothing in Change To and hit change all

Again - it's just a demonstration.

Because your have three sets of parentheses and therefore three groups.

And Peter's query is so much more efficient. There's a reason I normally sit out the GREP query questions. LOL

~Barb

Can you explain what you are trying to do here?

Your expression can be simplified to ^(\d+\.)+ so I'm not clear what you are looking to change.

Hi @dublove:

You have three groups. Your first group is the position locator: beginning of paragraph. Not sure why you are grouping it. InDesign will add a space for $1 if you leave it grouped. And then when nested groups, the encompassing group is counted first, and the nested one second. 

I want to search all the numbers

To simply match the numbers at the beginning of a paragraph to replace with something else here is one example. I suspect there are more efficient queries but I seem to be the only one here for the moment.

 ^(\d+)\.(\d+)?\.?(\d+)?\.?

~Barb