substring and regular expression

[borges2003xx@yahoo.it]

i have read
finding sublist

http://groups.google.it/group/comp.lang.python/browse_thread/thread/50b09a0aca285256/5156ada81fc9358a?hl=it#5156ada81fc9358a

the problem was in a string to find if we have two substring non
overlapping of lenght al least 4

it was done by
r=re.compile(r' (?P<seq>.{4,}). *(?P=seq)')

my knowhow on re is null
there is a way in re for ask if a substring is the reverse of the
otherone? First seq is second seq in reverse order

i appreciate all persons that can help me.
excuse my ignorance

[Kent Johnson]

Re: substring and regular expression

borges2003xx@ya hoo.it wrote:[color=blue]
> i have read
> finding sublist
> http://groups.google.it/group/comp.l...56ada81fc9358a
>
>
> the problem was in a string to find if we have two substring non
> overlapping of lenght al least 4
>
> it was done by
> r=re.compile(r' (?P<seq>.{4,}). *(?P=seq)')
>
> my knowhow on re is null
> there is a way in re for ask if a substring is the reverse of the
> otherone? First seq is second seq in reverse order[/color]

For a fixed-length substring you can use brute force. Here is a regex that matches a string containing a four-character substring and its reverse:
(?P<s1>.)(?P<s2 >.)(?P<s3>.)(?P <s4>.).*(?P=s4) (?P=s3)(?P=s2)( ?P=s1)

You might want to look at the Regular Expression HOW-TO document:

http://www.amk.ca/python/howto/regex/

Kent

[borges2003xx@yahoo.it]

Re: substring and regular expression

but in general is there a way to include in a re, in this example
something like...matches iff p , and q in which p==q[::-1] ? A way to
putting a small part of code of python in re? Thanx for your many helps

[Diez B.Roggisch]

Re: substring and regular expression

borges2003xx <at> yahoo.it <borges2003xx <at> yahoo.it> writes:
[color=blue]
>
> but in general is there a way to include in a re, in this example
> something like...matches iff p , and q in which p==q[::-1] ? A way to
> putting a small part of code of python in re? Thanx for your many helps[/color]

What you are after is a parser - there are plenty available. I prefer spark, but
pyparsing has gained lots of attention lately and is AFAIK the quasi-standard.

The problem you described is a classical instance of a so-called
context-free grammar.
There is absolutely _no_ way to teach regular expressions how to detect words
created based on a grammar of that kind. Really. It won't work.

So - either use a parser, or write a simple one yourself - the code above
qualifies as an attempt to do so.

Regards,

Diez

[borges2003xx@yahoo.it]

Re: substring and regular expression

can you be so kind to show me a small example in spark if u like for
remove all string with a substring of length 4 and the reverse of it?
i thank you a lot