regex query

Justin C · External Since: Mar 25, 2009 Posts: 3

I've been given a regex that matches white space between the last \w on
a line and the \n at the end of the line. The regex is:

\s+(?=\n)

I understand the \s+, what is going on with the (?=\n) ?

Thank you for any help you can give with this.

Justin.

--
Justin C, by the sea.

chris · External Since: Feb 08, 2005 Posts: 28

Justin C wrote:
> I've been given a regex that matches white space between the last \w on
> a line and the \n at the end of the line. The regex is:
>
> \s+(?=\n)
>
> I understand the \s+, what is going on with the (?=\n) ?
>
> Thank you for any help you can give with this.

It's the look-ahead. It finds the pattern by inspecting the surroundings
but without matching to them.

Owen Rees · External Since: Nov 17, 2004 Posts: 9

On Wed, 25 Mar 2009 15:47:38 -0000, Justin C
<justin.0903 DeleteThis @purestblue.com> wrote in <6017.49ca521a.9742@zem>:

>I understand the \s+, what is going on with the (?=\n) ?

It is "A zero-width positive lookahead assertion" to quote the perl
regex manual. It means that the match will succeed if the next thing is
a newline (in this case) but without including the newline in what is
matched.

You need to be a bit careful with these because not all regex systems
support them.

--
Owen Rees
[one of] my preferred email address[es] and more stuff can be
found at <http://www.users.waitrose.com/~owenrees/index.html>

Justin C · External Since: Mar 25, 2009 Posts: 3

On 2009-03-25, Owen Rees <orees DeleteThis @hotmail.com> wrote:
> On Wed, 25 Mar 2009 15:47:38 -0000, Justin C
><justin.0903 DeleteThis @purestblue.com> wrote in <6017.49ca521a.9742@zem>:
>
>>I understand the \s+, what is going on with the (?=\n) ?
>
> It is "A zero-width positive lookahead assertion" to quote the perl
> regex manual. It means that the match will succeed if the next thing is
> a newline (in this case) but without including the newline in what is
> matched.
>
> You need to be a bit careful with these because not all regex systems
> support them.

Thanks Owen, and Chris. I've just read about that in `perldoc perlre`,
that's complicated stuff! (at least for me). It does lead to a whole
bunch more reading because of references to perlvars with which I wasn't
familiar either.

I think I just need to remember that I can do 'look-ahead' (and behind)
matches in perl, and then look them up in the docs when I need them. I'm
never going to remember this stuff!

Anyway, thanks again.

Justin.

--
Justin C, by the sea.

chris · External Since: Feb 08, 2005 Posts: 28

Justin C wrote:
> On 2009-03-25, Owen Rees <orees RemoveThis @hotmail.com> wrote:
>> On Wed, 25 Mar 2009 15:47:38 -0000, Justin C
>> <justin.0903 RemoveThis @purestblue.com> wrote in <6017.49ca521a.9742@zem>:
>>
>>> I understand the \s+, what is going on with the (?=\n) ?
>> It is "A zero-width positive lookahead assertion" to quote the perl
>> regex manual. It means that the match will succeed if the next thing is
>> a newline (in this case) but without including the newline in what is
>> matched.
>>
>> You need to be a bit careful with these because not all regex systems
>> support them.
>
> Thanks Owen, and Chris. I've just read about that in `perldoc perlre`,
> that's complicated stuff! (at least for me). It does lead to a whole
> bunch more reading because of references to perlvars with which I wasn't
> familiar either.
>
> I think I just need to remember that I can do 'look-ahead' (and behind)
> matches in perl, and then look them up in the docs when I need them. I'm
> never going to remember this stuff!

That's what I do. I think I've only ever used it a couple of times, so
keeping it in my 'memory cache' is wasteful Wink

Geoffrey Clements · External Since: Apr 06, 2004 Posts: 226

"Justin C" <justin.0903 DeleteThis @purestblue.com> wrote in message
news:5598.49cb4d38.2b457@zem...
> On 2009-03-25, Owen Rees <orees DeleteThis @hotmail.com> wrote:
>> On Wed, 25 Mar 2009 15:47:38 -0000, Justin C
>><justin.0903 DeleteThis @purestblue.com> wrote in <6017.49ca521a.9742@zem>:
> I think I just need to remember that I can do 'look-ahead' (and behind)
> matches in perl, and then look them up in the docs when I need them. I'm
> never going to remember this stuff!
>

In reading this thread I discovered:
http://www.regular-expressions.info/

Looks like a very useful site.

--
Geoff

Bruce Stephens · External Since: Aug 19, 2004 Posts: 186

Justin C <justin.0903 RemoveThis @purestblue.com> writes:

[...]

> I think I just need to remember that I can do 'look-ahead' (and behind)
> matches in perl, and then look them up in the docs when I need them. I'm
> never going to remember this stuff!

I wouldn't worry too much. I suspect even frequent regexp users would
want to use them only very rarely.

Justin C · External Since: Mar 25, 2009 Posts: 3

On 2009-03-26, Geoffrey Clements <geoffrey.clementsNO.DeleteThis@SPAMbaesystems.com> wrote:
> "Justin C" <justin.0903.DeleteThis@purestblue.com> wrote in message
> news:5598.49cb4d38.2b457@zem...
>> On 2009-03-25, Owen Rees <orees.DeleteThis@hotmail.com> wrote:
>>> On Wed, 25 Mar 2009 15:47:38 -0000, Justin C
>>><justin.0903.DeleteThis@purestblue.com> wrote in <6017.49ca521a.9742@zem>:
>> I think I just need to remember that I can do 'look-ahead' (and behind)
>> matches in perl, and then look them up in the docs when I need them. I'm
>> never going to remember this stuff!
>>
>
> In reading this thread I discovered:
> http://www.regular-expressions.info/
>
> Looks like a very useful site.

Thanks for the pointer, it looks quite a useful site. I've book-marked
it for future reference.

Justin.

--
Justin C, by the sea.