Regular expression - first two alphabets of a string

ruby-talk
1

Hi All,

I learning regular expressions in Ruby and I want to do the following
thing with a strings

"A sting should not start with two a i.e aa"

example

"aa xyz" - wrong
"axyz" - is fine
"a xyz" is fine

please help.

Regards,

···

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

2

def matchaa (str)

  if str.match(/^aa/)
    return true
  else
     return false
  end
end

···

On 5/3/07, Ravi Singh <bobagent@gmail.com> wrote:

Hi All,

I learning regular expressions in Ruby and I want to do the following
thing with a strings

"A sting should not start with two a i.e aa"

example

"aa xyz" - wrong
"axyz" - is fine
"a xyz" is fine

please help.

Regards,

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

--
अभिजीत

[ written in http://www.paahijen.com/scratchpad ]

[ http://www.paahijen.com ]

3

Abhijit,

I tried the method for "aa xyz" and it return true actually it should
return false.

Regards,

···

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

4

This does not seem to be worth a method...

raise "Illegal String: #{str}" if /\Aaa/ =~ str

Kind regards

  robert

···

On 03.05.2007 19:33, Abhijit Gadgil wrote:

def matchaa (str)

if str.match(/^aa/)
   return true
else
    return false
end
end

5

Or use a negative lookahead:
irb(main):001:0> re = /^(?!aa)/
=> /^(?!aa)/
irb(main):002:0> "aa xyz".match(re)
=> nil
irb(main):003:0> "axyz".match(re)
=> #<MatchData:0xb7baf3e4>
irb(main):004:0> "a yz".match(re)
=> #<MatchData:0xb7bacb1c>

···

On 5/3/07, Abhijit Gadgil <gabhijit@gmail.com> wrote:

def matchaa (str)

  if str.match(/^aa/)
    return true
  else
     return false
  end
end

On 5/3/07, Ravi Singh <bobagent@gmail.com> wrote:
> Hi All,
>
> I learning regular expressions in Ruby and I want to do the following
> thing with a strings
>
> "A sting should not start with two a i.e aa"
>
> example
>
> "aa xyz" - wrong
> "axyz" - is fine
> "a xyz" is fine
>

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

6

Thanks Rick -ve lookahead helped.

Regards,

···

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

7

sorry! I misread your post! :frowning:

···

On 5/3/07, Ravi Singh <bobagent@gmail.com> wrote:

Abhijit,

I tried the method for "aa xyz" and it return true actually it should
return false.

Regards,

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

--
अभिजीत

[ written in http://www.paahijen.com/scratchpad ]

[ http://www.paahijen.com ]

8

What exactly do you gain by using lookahead instead of a normal match?

Kind regards

  robert

···

On 03.05.2007 20:30, Rick DeNatale wrote:

On 5/3/07, Abhijit Gadgil <gabhijit@gmail.com> wrote:

def matchaa (str)

  if str.match(/^aa/)
    return true
  else
     return false
  end
end

On 5/3/07, Ravi Singh <bobagent@gmail.com> wrote:
> Hi All,
>
> I learning regular expressions in Ruby and I want to do the following
> thing with a strings
>
> "A sting should not start with two a i.e aa"
>
> example
>
> "aa xyz" - wrong
> "axyz" - is fine
> "a xyz" is fine
>

Or use a negative lookahead:
irb(main):001:0> re = /^(?!aa)/
=> /^(?!aa)/
irb(main):002:0> "aa xyz".match(re)
=> nil
irb(main):003:0> "axyz".match(re)
=> #<MatchData:0xb7baf3e4>
irb(main):004:0> "a yz".match(re)
=> #<MatchData:0xb7bacb1c>

9

The OP was asking for an RE which matched anything which DIDN'T start with aa.

The proposal using if else got it backwards.

Of course you could also just negate the results of the match and use

!str.match(/aa/)

If you just wanted a test.

···

On 5/4/07, Robert Klemme <shortcutter@googlemail.com> wrote:

On 03.05.2007 20:30, Rick DeNatale wrote:

>
> Or use a negative lookahead:
> irb(main):001:0> re = /^(?!aa)/
> => /^(?!aa)/
> irb(main):002:0> "aa xyz".match(re)
> => nil
> irb(main):003:0> "axyz".match(re)
> => #<MatchData:0xb7baf3e4>
> irb(main):004:0> "a yz".match(re)
> => #<MatchData:0xb7bacb1c>

What exactly do you gain by using lookahead instead of a normal match?

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

IPMS/USA Region 12 Coordinator
http://ipmsr12.denhaven2.com/

Visit the Project Mercury Wiki Site
http://www.mercuryspacecraft.com/

10

Robert Klemme wrote:

···

On 03.05.2007 20:30, Rick DeNatale wrote:

On 5/3/07, Ravi Singh <bobagent@gmail.com> wrote:
> "axyz" - is fine

irb(main):004:0> "a yz".match(re)
=> #<MatchData:0xb7bacb1c>

What exactly do you gain by using lookahead instead of a normal match?

Kind regards

  robert

When Rick directed me to use -ve lookahead I reach the following page
and it made the point clear.

Regards,

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