Using Groups inside Braces with Regular Expressions

[Chris]

I'm trying to delimit sentences in a block of text by defining the
end-of-sentence marker as a period followed by a space followed by an
uppercase letter or end-of-string.

I'd imagine the regex for that would look something like:
[^(?:[A-Z]|$)]\.\s+(?=[A-Z]|$)

However, Python keeps giving me an "unbalanced parenthesis" error for
the [^] part. If this isn't valid regex syntax, how else would I match
a block of text that doesn't the delimiter pattern?

Thanks,
Chris

[MRAB]

Re: Using Groups inside Braces with Regular Expressions

On Jul 14, 12:05 am, Chris <chriss...@gmai l.comwrote:

I'm trying to delimit  sentences in a block of text by defining the
end-of-sentence marker as a period followed by a space followed by an
uppercase letter or end-of-string.
>
I'd imagine the regex for that would look something like:
[^(?:[A-Z]|$)]\.\s+(?=[A-Z]|$)
>
However, Python keeps giving me an "unbalanced parenthesis" error for
the [^] part. If this isn't valid regex syntax, how else would I match
a block of text that doesn't the delimiter pattern?
>

What is the [^(?:[A-Z]|$)] part meant to be doing? Is it meant to be
matching everything up to the end of the sentence?

[...] is a character class, so Python is parsing the character class
as:

[^(?:[A-Z]|$)]
^^^^^^^^^^

[Chris]

Re: Using Groups inside Braces with Regular Expressions

On Jul 13, 8:14 pm, MRAB <goo...@mrabarn ett.plus.comwro te:

On Jul 14, 12:05 am, Chris <chriss...@gmai l.comwrote:I'm trying to delimit sentences in a block of text by defining the

end-of-sentence marker as a period followed by a space followed by an
uppercase letter or end-of-string.

>

I'd imagine the regex for that would look something like:
[^(?:[A-Z]|$)]\.\s+(?=[A-Z]|$)

>

However, Python keeps giving me an "unbalanced parenthesis" error for
the [^] part. If this isn't valid regex syntax, how else would I match
a block of text that doesn't the delimiter pattern?

>
What is the [^(?:[A-Z]|$)] part meant to be doing? Is it meant to be
matching everything up to the end of the sentence?
>
[...] is a character class, so Python is parsing the character class
as:
>
[^(?:[A-Z]|$)]
^^^^^^^^^^

It was meant to include everything except the end-of-sentence pattern.
However, I just realized that I can simply replace it with ".*?"

[John Machin]

Re: Using Groups inside Braces with Regular Expressions

On Jul 14, 9:05 am, Chris <chriss...@gmai l.comwrote:

Misleading subject.

[] brackets or "square brackets"
{} braces or "curly brackets"
() parentheses or "round brackets"

I'm trying to delimit sentences in a block of text by defining the
end-of-sentence marker as a period followed by a space followed by an
uppercase letter or end-of-string.

.... which has at least two problems:

(1) You are insisting on at least one space between the period and the
end-of-string (this can be overcome, see later).
(2) Periods are often dropped in after abbreviations and contractions
e.g. "Mr. Geo. Smith". You will get three "sentences" out of that.

>
I'd imagine the regex for that would look something like:
[^(?:[A-Z]|$)]\.\s+(?=[A-Z]|$)
>
However, Python keeps giving me an "unbalanced parenthesis" error for
the [^] part.

It's nice to know that Python is consistent with its error messages.

If this isn't valid regex syntax,

If? It definitely isn't valid syntax. The brackets should delimit a
character class. You are trying to cram a somewhat complicated
expression into a character class, or you should be using parentheses.
However it's a bit hard to determine what you really meant that part
of the pattern to achieve.

how else would I match
a block of text that doesn't the delimiter pattern?

Start from the top down:
A sentence is:
anything (with some qualifications)
followed by (but not including):
a period
followed by
either
1 or more whitespaces then a capital letter
or
0 or more whitespaces then end-of-string

So something like this might do the trick:

>>sep = re.compile(r'\. (?:\s+(?=[A-Z])|\s*(?=\Z))')
>>sep.split('He llo. Mr. Chris X\nis here.\nIP addr 1.2.3.4. ')

['Hello', 'Mr', 'Chris X\nis here', 'IP addr 1.2.3.4', '']