Regexp to match strings that _don't_ being with a string

ruby-talk
1

I would like to write a regexp that will match a string that does NOT
start with a specified set of characters.

For example,

Given:

xyz123
asldfhsl
xyk2345

and assume that I want to see only strings that don't start with "xyz"
(so in this case, the last 2 in the list).

I tried /^[^(xyz)]/ but I don't trust it. I don't think the grouping
will take inside the character class.

Do I need a negative lookahead assertion?

Thanks,
Wes

···

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Posted via http://www.ruby-forum.com/.

2

I tried /^[^(xyz)]/ but I don't trust it. I don't think the grouping
will take inside the character class.

The [^(xyz)] creates a negative character class, so your regex would
match any string that started with a character not in the given set.
Not what you really want

Do I need a negative lookahead assertion?

That would be a simple solution: /^(?!xyz)/ which will match when
the beginning of the line/string is not followed by 'xyz'.

andrew

···

On Wed, 29 Mar 2006 03:24:36 +0900, Wes Gamble <weyus@att.net> wrote:

--
Andrew L. Johnson http://www.siaris.net/
      What have you done to the cat? It looks half-dead.
          -- Schroedinger's wife

3

What about:

! s =~ /^xyz/

There is a good discussion of doing exactly this with negative
look-aheads in 'man perlre'. It's... ugly.

4

In my example, won't /^(?!xyz)/ also match

29384723xyz02342

which is a little more than I want?

WG

Andrew Johnson wrote:

···

On Wed, 29 Mar 2006 03:24:36 +0900, Wes Gamble <weyus@att.net> wrote:

I tried /^[^(xyz)]/ but I don't trust it. I don't think the grouping
will take inside the character class.

The [^(xyz)] creates a negative character class, so your regex would
match any string that started with a character not in the given set.
Not what you really want

Do I need a negative lookahead assertion?

That would be a simple solution: /^(?!xyz)/ which will match when
the beginning of the line/string is not followed by 'xyz'.

andrew

--
Posted via http://www.ruby-forum.com/\.

5

Ruby has a doesn't match operator for this:

   s !~ /^xyz/

James Edward Gray II

···

On Mar 28, 2006, at 12:40 PM, Chris Alfeld wrote:

! s =~ /^xyz/

6

Hey that's too easy!!!

Thanks James.

I'm liking Ruby more and more :).

Wes

James Gray wrote:

···

On Mar 28, 2006, at 12:40 PM, Chris Alfeld wrote:

! s =~ /^xyz/

Ruby has a doesn't match operator for this:

   s !~ /^xyz/

James Edward Gray II

--
Posted via http://www.ruby-forum.com/\.

7

Uhm, maybe I've misunderstood (wouldn't be the first time) -- I thought you
wanted to match strings that did not begin with 'xyz' ... and as far as I
can tell, "29384723xyz02342" does not start with 'xyz'.

  while DATA.gets
    print if ~/^(?!xyz)/
  end
  __END__
  xyefoo
  xyzpfoo
  asdfsdf
  1230xyzasdf

produces:

  xyefoo
  asdfsdf
  1230xyzasdf

puzzled,
andrew

···

On Wed, 29 Mar 2006 03:43:51 +0900, Wes Gamble <weyus@att.net> wrote:

In my example, won't /^(?!xyz)/ also match

29384723xyz02342

which is a little more than I want?

--
Andrew L. Johnson http://www.siaris.net/
      People seem not to see that their opinion of the world is
      also a confession of character.
          -- Ralph Waldo Emerson; The Conduct of Life, 1860

8

Wes Gamble wrote:

Hey that's too easy!!!

Thanks James.

I'm liking Ruby more and more :).

Wes

James Gray wrote:
  

! s =~ /^xyz/
      

Ruby has a doesn't match operator for this:

   s !~ /^xyz/

James Edward Gray II
    

Awesome.

-Justin

···

On Mar 28, 2006, at 12:40 PM, Chris Alfeld wrote:

9

Andrew,

That works fine.

In actuality, I do need the ability to pass one regex to do the job into
another utility that will use it to operate on an array of strings.

So although !~ is cool, I really didn't want to have to iterate through
the strings myself.

Thanks,
Wes

Andrew Johnson wrote:

···

On Wed, 29 Mar 2006 03:43:51 +0900, Wes Gamble <weyus@att.net> wrote:

In my example, won't /^(?!xyz)/ also match

29384723xyz02342

which is a little more than I want?

Uhm, maybe I've misunderstood (wouldn't be the first time) -- I thought
you
wanted to match strings that did not begin with 'xyz' ... and as far as
I
can tell, "29384723xyz02342" does not start with 'xyz'.

  while DATA.gets
    print if ~/^(?!xyz)/
  end
  __END__
  xyefoo
  xyzpfoo
  asdfsdf
  1230xyzasdf

produces:

  xyefoo
  asdfsdf
  1230xyzasdf

puzzled,
andrew

--
Posted via http://www.ruby-forum.com/\.

10

I have a new wrinkle.

Now I want to match any line that doesn't have "xyz" or "abc" at the
beginning of the line.

Is there a way to "AND" together the input to the negative lookahead
assertion?

Wes

Wes Gamble wrote:

···

Andrew,

That works fine.

In actuality, I do need the ability to pass one regex to do the job into
another utility that will use it to operate on an array of strings.

So although !~ is cool, I really didn't want to have to iterate through
the strings myself.

Thanks,
Wes

Andrew Johnson wrote:

On Wed, 29 Mar 2006 03:43:51 +0900, Wes Gamble <weyus@att.net> wrote:

In my example, won't /^(?!xyz)/ also match

29384723xyz02342

which is a little more than I want?

Uhm, maybe I've misunderstood (wouldn't be the first time) -- I thought
you
wanted to match strings that did not begin with 'xyz' ... and as far as
I
can tell, "29384723xyz02342" does not start with 'xyz'.

  while DATA.gets
    print if ~/^(?!xyz)/
  end
  __END__
  xyefoo
  xyzpfoo
  asdfsdf
  1230xyzasdf

produces:

  xyefoo
  asdfsdf
  1230xyzasdf

puzzled,
andrew

--
Posted via http://www.ruby-forum.com/\.

11

Wes Gamble <weyus@att.net> writes:

Andrew,

That works fine.

In actuality, I do need the ability to pass one regex to do the job into
another utility that will use it to operate on an array of strings.

So although !~ is cool, I really didn't want to have to iterate through
the strings myself.

Thanks,
Wes

I always wanted to propose a ~ operator for Regexps. :wink:
The current behavior is next to useless.

It wouldn't be difficult to add... also, see lib/eregex.rb.

···

--
Christian Neukirchen <chneukirchen@gmail.com> http://chneukirchen.org

12

Wes Gamble wrote:

I have a new wrinkle.

Now I want to match any line that doesn't have "xyz" or "abc" at the
beginning of the line.

Is there a way to "AND" together the input to the negative lookahead
assertion?

For lookaheads, you can get AND by concatenating:

irb(main):001:0> /^(?!abc)(?!xyz)/ =~ "abc"
=> nil
irb(main):002:0> /^(?!abc)(?!xyz)/ =~ " abc"
=> 0
irb(main):003:0> /^(?!abc)(?!xyz)/ =~ "xyz"
=> nil
irb(main):004:0> /^(?!abc)(?!xyz)/ =~ " xyz"
=> 0

···

--
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

13

Thanks, that makes sense since the lookahead doesn't "consume" right?

WG
Joel VanderWerf wrote:

···

Wes Gamble wrote:

I have a new wrinkle.

Now I want to match any line that doesn't have "xyz" or "abc" at the
beginning of the line.

Is there a way to "AND" together the input to the negative lookahead
assertion?

For lookaheads, you can get AND by concatenating:

irb(main):001:0> /^(?!abc)(?!xyz)/ =~ "abc"
=> nil
irb(main):002:0> /^(?!abc)(?!xyz)/ =~ " abc"
=> 0
irb(main):003:0> /^(?!abc)(?!xyz)/ =~ "xyz"
=> nil
irb(main):004:0> /^(?!abc)(?!xyz)/ =~ " xyz"
=> 0

--
Posted via http://www.ruby-forum.com/\.

14

Wes Gamble wrote:

Thanks, that makes sense since the lookahead doesn't "consume" right?

Right. In this case an alternation works, too:

p %w{abcd xyz as}.map {|s| /^(?!abc|xyz)/=~s}
=> [nil, nil, 0]

De Morgan's Law comes to mind: not a and not b <=> not(a or b) :slight_smile:

Kind regards

  robert

···

WG
Joel VanderWerf wrote:

Wes Gamble wrote:

I have a new wrinkle.

Now I want to match any line that doesn't have "xyz" or "abc" at the beginning of the line.

Is there a way to "AND" together the input to the negative lookahead assertion?

For lookaheads, you can get AND by concatenating:

irb(main):001:0> /^(?!abc)(?!xyz)/ =~ "abc"
=> nil
irb(main):002:0> /^(?!abc)(?!xyz)/ =~ " abc"
=> 0
irb(main):003:0> /^(?!abc)(?!xyz)/ =~ "xyz"
=> nil
irb(main):004:0> /^(?!abc)(?!xyz)/ =~ " xyz"
=> 0