Regular expression - first two alphabets of a string

[Ravi Singh]

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,

 

[Abhijit Gadgil]

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[Ravi Singh]

Abhijit,

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

Regards,

 

[Robert Klemme]

def matchaa (str)

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

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

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

Kind regards

robert

 

[Rick DeNatale]

def matchaa (str)

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

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>

 

[Ravi Singh]

Thanks Rick -ve lookahead helped.

Regards,

 

[Abhijit Gadgil]

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[Robert Klemme]

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?

Kind regards

robert

 

[Rick DeNatale]

On 03.05.2007 20:30, Rick DeNatale wrote:

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

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.



--
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/

 

[Ravi Singh]

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.

http://www.regular-expressions.info/lookaround.html

Regards,