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Groups > comp.lang.python > #98269
| From | Ben Finney <ben+python@benfinney.id.au> |
|---|---|
| Newsgroups | comp.lang.python |
| Subject | Re: Regular expressions |
| Date | 2015-11-05 14:03 +1100 |
| Message-ID | <mailman.41.1446692637.16136.python-list@python.org> (permalink) |
| References | (8 earlier) <n1bs2g$376$1@dont-email.me> <56397a18$0$11094$c3e8da3@news.astraweb.com> <56397FC6.9040700@gmail.com> <mailman.6.1446627436.16136.python-list@python.org> <563abee1$0$1614$c3e8da3$5496439d@news.astraweb.com> |
Steven D'Aprano <steve@pearwood.info> writes:
> On Wed, 4 Nov 2015 07:57 pm, Peter Otten wrote:
>
> > I tried Tim's example
> >
> > $ seq 5 | grep '1*'
> > 1
> > 2
> > 3
> > 4
> > 5
> > $
>
> I don't understand this. What on earth is grep matching? How does "4"
> match "1*"?
You can experiment with regular expressions to find out. Here's a link
to the RegExr tool for the above pattern <URL:http://regexr.com/3c4ot>.
Matching patterns can include specifications meaning “match some number
of the preceding segment”, with the ‘{n,m}’ notation. That means “match
at least n, and at most m, occurrences of the preceding segment”. Either
‘n’ or ‘m’ can be omitted, meaning “at least 0” and “no maximum”
respectively.
Those are quite useful, so there are shortcuts for the most common
cases: ‘?’ is a short cut for ‘{0,1}’, ‘*’ is a short cut for ‘{0,}’,
and ‘+’ is a short cut for ‘{1,}’.
In this case, ‘*’ is a short cut for ‘{0,}’ meaning “match 0 or more
occurences of the preceding segment”. The segment here is the atom ‘1’.
Since ‘1*’ is the entirety of the pattern, the pattern can match zero
characters, anywhere within any string. So, it matches every possible
string.
To match (some atom) 1 or more times, ‘+’ is a short cut for ‘(1,}’
meaning “match 1 or more occurrences of the preceding segment”.
--
\ 學而不思則罔，思而不學則殆。 (To study and not think is a waste. |
`\ To think and not study is dangerous.) |
_o__) —孔夫子 Confucius (551 BCE – 479 BCE) |
Ben Finney
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Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-02 20:09 -0500
Re: Regular expressions MRAB <python@mrabarnett.plus.com> - 2015-11-03 01:19 +0000
Re: Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-02 22:17 -0500
Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-02 20:42 -0600
Re: Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-02 22:17 -0500
Re: Regular expressions Joel Goldstick <joel.goldstick@gmail.com> - 2015-11-02 22:58 -0500
Re: Regular expressions rurpy@yahoo.com - 2015-11-02 20:23 -0800
Re: Regular expressions Michael Torrie <torriem@gmail.com> - 2015-11-02 21:38 -0700
Re: Regular expressions rurpy@yahoo.com - 2015-11-03 16:33 -0800
Re: Regular expressions Michael Torrie <torriem@gmail.com> - 2015-11-03 19:04 -0700
Re: Regular expressions Dan Sommers <dan@tombstonezero.net> - 2015-11-04 02:55 +0000
Re: Regular expressions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-11-04 14:23 +1100
Re: Regular expressions Michael Torrie <torriem@gmail.com> - 2015-11-03 20:47 -0700
Re: Regular expressions Grant Edwards <invalid@invalid.invalid> - 2015-11-04 13:27 +0000
Re: Regular expressions Nobody <nobody@nowhere.invalid> - 2015-11-04 05:05 +0000
Re: Regular expressions Peter Otten <__peter__@web.de> - 2015-11-04 09:57 +0100
Re: Regular expressions Steven D'Aprano <steve@pearwood.info> - 2015-11-05 13:28 +1100
Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-04 20:48 -0600
Re: Regular expressions Ben Finney <ben+python@benfinney.id.au> - 2015-11-05 14:03 +1100
Re: Regular expressions Peter Otten <__peter__@web.de> - 2015-11-05 09:33 +0100
Re: Regular expressions Steven D'Aprano <steve@pearwood.info> - 2015-11-05 23:05 +1100
Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-05 08:00 -0600
Re: Regular expressions Albert van der Horst <albert@spenarnc.xs4all.nl> - 2015-11-05 13:39 +0000
Re: Regular expressions Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-11-04 08:00 -0500
Re: Regular expressions Michael Torrie <torriem@gmail.com> - 2015-11-04 08:13 -0700
Re: Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-04 18:00 -0500
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 16:24 -0800
Re: Regular expressions Steven D'Aprano <steve@pearwood.info> - 2015-11-05 13:24 +1100
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 21:59 -0800
Re: Regular expressions Christian Gollwitzer <auriocus@gmx.de> - 2015-11-05 09:18 +0100
Re: Regular expressions rurpy@yahoo.com - 2015-11-06 11:52 -0800
Re: Regular expressions Christian Gollwitzer <auriocus@gmx.de> - 2015-11-06 21:36 +0100
Re: Regular expressions Larry Martell <larry.martell@gmail.com> - 2015-11-06 15:42 -0500
Re: Regular expressions Chris Angelico <rosuav@gmail.com> - 2015-11-05 11:34 +1100
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 22:27 -0800
Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-04 09:42 -0600
Re: Regular expressions Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-11-05 20:55 +1300
Re: Regular expressions Chris Angelico <rosuav@gmail.com> - 2015-11-05 19:06 +1100
What does “grep” stand for? (was: Regular expressions) Ben Finney <ben+python@benfinney.id.au> - 2015-11-05 05:24 +1100
Re: What does “grep” stand for? Christian Gollwitzer <auriocus@gmx.de> - 2015-11-04 20:38 +0100
Re: What does “grep” stand for? Chris Angelico <rosuav@gmail.com> - 2015-11-05 11:42 +1100
Re: What does “grep” stand for? Christian Gollwitzer <auriocus@gmx.de> - 2015-11-05 08:32 +0100
Re: What does “grep” stand for? Chris Angelico <rosuav@gmail.com> - 2015-11-05 19:00 +1100
Re: What does “grep” stand for? Random832 <random832@fastmail.com> - 2015-11-05 10:19 -0500
Re: What does “grep” stand for? Grant Edwards <invalid@invalid.invalid> - 2015-11-05 18:29 +0000
Re: What does “grep” stand for? Random832 <random832@fastmail.com> - 2015-11-05 14:56 -0500
Re: What does “grep” stand for? Grant Edwards <invalid@invalid.invalid> - 2015-11-05 20:19 +0000
Re: What does “grep” stand for? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-11-05 20:18 -0500
Re: What does “grep” stand for? Larry Hudson <orgnut@yahoo.com> - 2015-11-05 19:36 -0800
Re: What does “grep” stand for? Dan Sommers <dan@tombstonezero.net> - 2015-11-06 05:31 +0000
Re: What does “grep” stand for? William Ray Wing <wrw@mac.com> - 2015-11-06 08:25 -0500
Re: What does “grep” stand for? Larry Hudson <orgnut@yahoo.com> - 2015-11-06 19:21 -0800
Re: What does ???grep??? stand for? Grant Edwards <invalid@invalid.invalid> - 2015-11-06 14:15 +0000
Re: What does ???grep??? stand for? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-11-06 20:03 -0500
Re: What does “grep” stand for? (was: Regular expressions) Tim Chase <python.list@tim.thechases.com> - 2015-11-04 13:05 -0600
Re: Regular expressions Terry Reedy <tjreedy@udel.edu> - 2015-11-04 18:08 -0500
Re: Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-04 18:29 -0500
Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-03 21:12 -0600
Re: Regular expressions Chris Angelico <rosuav@gmail.com> - 2015-11-04 14:26 +1100
Re: Regular expressions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-11-04 14:48 +1100
Re: Regular expressions Christian Gollwitzer <auriocus@gmx.de> - 2015-11-04 08:21 +0100
Re: Regular expressions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-11-04 19:47 +1100
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 06:43 -0800
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 06:38 -0800
Re: Regular expressions Chris Angelico <rosuav@gmail.com> - 2015-11-05 01:52 +1100
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 16:13 -0800
Re: Regular expressions Chris Angelico <rosuav@gmail.com> - 2015-11-05 11:33 +1100
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 21:42 -0800
Re: Regular expressions Steven D'Aprano <steve@pearwood.info> - 2015-11-05 13:26 +1100
Re: Regular expressions Ben Finney <ben+python@benfinney.id.au> - 2015-11-05 14:07 +1100
Re: Regular expressions rurpy@yahoo.com - 2015-11-04 21:54 -0800
Re: Regular expressions Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-05 10:14 +0100
Re: Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-04 18:02 -0500
Re: Regular expressions Steven D'Aprano <steve@pearwood.info> - 2015-11-05 11:54 +1100
Re: Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-05 10:07 -0500
Re: Regular expressions rurpy@yahoo.com - 2015-11-06 12:46 -0800
Re: Regular expressions Steven D'Aprano <steve@pearwood.info> - 2015-11-03 18:15 +1100
Re: Regular expressions Nick Sarbicki <nick.a.sarbicki@gmail.com> - 2015-11-03 08:43 +0000
Re: Regular expressions rurpy@yahoo.com - 2015-11-03 16:22 -0800
Re: Regular expressions Denis McMahon <denismfmcmahon@gmail.com> - 2015-11-03 12:38 +0000
Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-03 05:53 -0600
Re: Regular expressions Joel Goldstick <joel.goldstick@gmail.com> - 2015-11-03 10:34 -0500
Re: Regular expressions Seymore4Head <Seymore4Head@Hotmail.invalid> - 2015-11-03 11:10 -0500
Re: Regular expressions Chris Angelico <rosuav@gmail.com> - 2015-11-04 03:20 +1100
Re: Regular expressions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-11-04 14:35 +1100
Re: Regular expressions Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-04 12:41 +0100
Re: Regular expressions Grant Edwards <invalid@invalid.invalid> - 2015-11-03 14:56 +0000
Re: Regular expressions Michael Torrie <torriem@gmail.com> - 2015-11-02 20:51 -0700
Re: Regular expressions rurpy@yahoo.com - 2015-11-02 20:23 -0800
Re: Regular expressions Michael Torrie <torriem@gmail.com> - 2015-11-02 21:33 -0700
Re: Regular expressions Robin Koch <robin.koch@t-online.de> - 2015-11-03 23:58 +0100
Re: Regular expressions Peter Otten <__peter__@web.de> - 2015-11-03 10:25 +0100
Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-03 05:50 -0600
Re: Regular expressions Peter Otten <__peter__@web.de> - 2015-11-03 15:00 +0100
Re: Regular expressions Jussi Piitulainen <harvesting@makes.email.invalid> - 2015-11-03 17:12 +0200
Irregular last line in a text file, was Re: Regular expressions Peter Otten <__peter__@web.de> - 2015-11-03 16:35 +0100
Re: Irregular last line in a text file, was Re: Regular expressions Jussi Piitulainen <harvesting@makes.email.invalid> - 2015-11-03 18:42 +0200
Re: Irregular last line in a text file, was Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-03 10:56 -0600
Re: Irregular last line in a text file, was Re: Regular expressions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-11-04 14:39 +1100
Re: Irregular last line in a text file, was Re: Regular expressions Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-11-04 10:07 +0000
Re: Irregular last line in a text file, was Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-04 09:33 -0600
Re: Irregular last line in a text file, was Re: Regular expressions Peter Otten <__peter__@web.de> - 2015-11-03 18:44 +0100
Re: Irregular last line in a text file, was Re: Regular expressions Ian Kelly <ian.g.kelly@gmail.com> - 2015-11-03 11:33 -0700
Re: Irregular last line in a text file, was Re: Regular expressions Ian Kelly <ian.g.kelly@gmail.com> - 2015-11-03 11:39 -0700
Re: Irregular last line in a text file, was Re: Regular expressions Tim Chase <python.list@tim.thechases.com> - 2015-11-03 13:45 -0600
Re: Irregular last line in a text file, was Re: Regular expressions Grant Edwards <invalid@invalid.invalid> - 2015-11-03 22:15 +0000
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