Re: inverse regexp

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Re: inverse regexp

Jason Breslau

26 Jul 2008 17:56

"Bruno Luong" <b.luong@fogale.fr.findthecountry> wrote in
message <g6eq9v$cii$1@fred.mathworks.com>...

> Impressive! This kind of code reminds me now why I never get
> around to understand fully regexp syntax.
>
> Bruno

I'm sorry if I turned you off of it. Regular Expressions
are really fun, and can be useful, too, if used properly.

The recursive example I used was modified from an example in
Jeffrey Friedl's "Mastering Regular Expressions", which is a
fantastic book on regular expressions. I couldn't recommend
it more: http://regex.info/

-=>J

Bruno Luong

26 Jul 2008 09:21

Jason Breslau <tendiamonds@mathworks.com> wrote in message

> >> str = 'Johan and Mary';
> >> rex = '(?<husband>[a-z]([aeiou](?=[a-z])[^aeiou])*) and (?<wife>\S+)';
> >> s = regexpi(str,rex,'names');
> >> levelN = '$([^()]|(??@levelN))*$';
> >> regexprep(rex, '$\?<(\w*)>([^()]|(??@levelN))*$', '${s.($1)}')

Impressive! This kind of code reminds me now why I never get
around to understand fully regexp syntax.

Bruno

Jason Breslau

26 Jul 2008 02:49

Hi Jean-Yves,

Interesting problem.

To solve the simple case, in which there are only named tokens and
literal strings, you can use regexprep to reconstruct the match, but as
Ashish pointed out, you will need 'split' to recreate the portions of
the initial string that are not matched.

To make things simpler, you can restrict your pattern to named tokens
that do not contain nested parentheses:

>> str = 'Johan and Mary';
>> rex = '(?<husband>\S+) and (?<wife>\S+)';
>> s = regexp(str,rex,'names');
>> regexprep(rex, '$\?<(\w*)>.*?$', '${s.($1)}')

ans =

Johan and Mary

See how this fails with a pattern using nested parentheses:

>> str = 'Johan and Mary';
>> rex = '(?<husband>[a-z]([aeiou](?=[a-z])[^aeiou])*) and (?<wife>\S+)';
>> s = regexpi(str,rex,'names')

s =

husband: 'Johan'
wife: 'Mary'

>> regexprep(rex, '$\?<(\w*)>.*?$', '${s.($1)}')

ans =

Johan[^aeiou])*) and Mary

To handle nested parentheses, you need to use a recursive pattern:

>> str = 'Johan and Mary';
>> rex = '(?<husband>[a-z]([aeiou](?=[a-z])[^aeiou])*) and (?<wife>\S+)';
>> s = regexpi(str,rex,'names');
>> levelN = '$([^()]|(??@levelN))*$';
>> regexprep(rex, '$\?<(\w*)>([^()]|(??@levelN))*$', '${s.($1)}')

ans =

Johan and Mary

-=>J

Jean-Yves Tinevez

25 Jul 2008 17:46

I was wondering if anyone wrote or met a function that was
inverting regexp.

For instance, let's suppose I want to extract tokens out a a
string. I can do that with regexp:

>> str = 'Johan and Mary';
>> rex = '(?<husband>\S+) and (?<wife>\S+)';
>> s = regexp(str,rex,'names')

s =

husband: 'Johan'
wife: 'Mary'

Now I would like to do the inverse transformation, and have
a function that would do this:

n = iregexp(rex,s)

and would output

n = 'Johan and Mary'

Has anyone ever met something like this? Or would have an
idea on how to do it?
jy

Re: inverse regexp

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