[Gambas-user] Help needed from regexp gurus
Fernando Cabral
fernandojosecabral at ...626...
Sun Jun 18 01:57:21 CEST 2017
Tobi
One more thing about the way I wish it could work (I remember having done
this in C perhaps 30 years ago). The pseudo-code bellow is pretty
schematic, but I think it will clarify the issue.
Let p and l be arrays of integers and s be the string "abc defg hijkl"
So, after traversing the string we would have the following result:
p[0] = offset of "a" (0)
l[0] = length of "abc" (3)
p[1] = offset of "d" (4)
l[1] = lenght of "defg" (4)
p[2] = offset of "h" (9)
l[2] = lenght of "hijkl" (5).
After this, each word could be retrieved in the following manner:
for i = 0 to 2
print mid(s, p[i], l[i])
next
I think this would be the most efficient way to do it. But I can't find how
to do it in Gambas using Regex.
Regards
- fernando
2017-06-17 18:06 GMT-03:00 Tobias Boege <taboege at ...626...>:
> On Sat, 17 Jun 2017, Fernando Cabral wrote:
> > Still beating my head against the wall due to my lack of knowledge about
> > the PCRE methods and properties... Because of this, I have progressed not
> > only very slowly but also -- I fell -- in a very inelegant way. So
> perhaps
> > you guys who are more acquainted with PCRE might be able to hint me on a
> > better solution.
> >
> > I want to search a long string that can contain a sentence, a paragraph
> or
> > even a full text. I wanna find and isolate every word it contains. A word
> > is defined as any sequence of alphabetic characters followed by a
> > non-alphatetic character.
> >
>
> The Mathematician in me can't resist to point this out: you hopefully
> wanted
> to define "word in a string" as "a *longest* sequence of alphabetic
> characters
> followed by a non-alphabetic character (or the end of the string)". Using
> your
> definition above, the words in "abc:" would be "c", "bc" and "abc", whereas
> you probably only wanted "abc" (the longest of those).
>
> > The sample code bellow does work, but I don't feel it is as elegant and
> as
> > fast as it could and should be. Especially the way I am traversing the
> > string from the beginning to the end. It looks awkward and slow. There
> must
> > be a more efficient way, like working only with offsets and lengths
> instead
> > of copying the string again and again.
> >
>
> You think worse of String.Mid() than it deserves, IMHO. Gambas strings
> are triples of a pointer to some data, a start index and a length, and
> the built-in string functions take care not to copy a string when it's
> not necessary. The plain Mid$() function (dealing with ASCII strings only)
> is implemented as a constant-time operation which simply takes your input
> string and adjusts the start index and length to give you the requested
> portion of the string. The string doesn't even have to be read, much less
> copied, to do this.
>
> Now, the String.Mid() function is somewhat more complicated, because
> UTF-8 strings have variable-width characters, which makes it difficult
> to map byte indices to character positions. To implement String.Mid(),
> your string has to be read, but, again, not copied.
>
> Extracting a part of a string is a non-destructive operation in Gambas
> and no copying takes place. (Concatenating strings, on the other hand,
> will copy.) So, there is some reading overhead (if you need UTF-8 strings),
> but it's smaller than you probably thought.
>
> > Dim Alphabetics as string "abc...zyzABC...ZYZ"
> > Dim re as RegExp
> > Dim matches as String []
> > Dim RawText as String
> >
> > re.Compile("([" & Alphabetics & "]+?)([^" & Alphabetics & "]+)",
> > RegExp.utf8)
> > RawText = "abc12345def ghi jklm mno p1"
> >
> > Do While RawText
> > re.Exec(RawText)
> > matches.add(re[1].text)
> > RawText = String.Mid(RawText, String.Len(re.text) + 1)
> > Loop
> >
> > For i = 0 To matches.Count - 1
> > Print matches[i]
> > Next
> >
> >
> > Above code correctly finds "abc, def, ghi, jlkm, mno, p". But the tricks
> I
> > have used are cumbersome (like advancing with string.mid() and resorting
> to
> > re[1].text and re.text.
> >
>
> Well, I think you can't use PCRE alone to solve your problem, if you want
> to capture a variable number of words in your submatches. I did a bit of
> reading and from what I gather [1][2] capturing group numbers are assigned
> based on the verbatim regular expression, i.e. the number of submatches
> you can receive is limited by the number of "(...)" constructs in your
> expression; and the (otherwise very nifty) recursion operator (?R) does
> not give you an unlimited number of capturing groups, sadly.
>
> Anyway, I think by changing your regular expression, you can let PCRE take
> care of the string advancement, like so:
>
> 1 #!/usr/bin/gbs3
> 2
> 3 Use "gb.pcre"
> 4
> 5 Public Sub Main()
> 6 Dim r As New RegExp
> 7 Dim s As string
> 8
> 9 r.Compile("([[:alpha:]]+)[[:^alpha:]]+(.*$)", RegExp.UTF8)
> 10 s = "abc12345def ghi jklm mno p1"
> 11 Print "Subject:";; s
> 12 Do
> 13 r.Exec(s)
> 14 If r.Offset = -1 Then Break
> 15 Print " ->";; r[1].Text
> 16 s = r[2].Text
> 17 Loop While s
> 18 End
>
> Output:
>
> Subject: abc12345def ghi jklm mno p1
> -> abc
> -> def
> -> ghi
> -> jklm
> -> mno
> -> p
>
> But, I think, this is less efficient than using String.Mid(). The trailing
> group (.*$) _may_ make the PCRE library read the entire subject every time.
> And I believe gb.pcre will copy your submatch string when returning it.
> If you care deeply about this, you'll have to trace the code in gb.pcre
> and main/gbx (the interpreter) to see what copies strings and what doesn't.
>
> Regards,
> Tobi
>
> [1] http://www.regular-expressions.info/recursecapture.html (Capturing
> Groups Inside Recursion or Subroutine Calls)
> [2] http://www.rexegg.com/regex-recursion.html (Groups Contents and
> Numbering in Recursive Expressions)
>
> --
> "There's an old saying: Don't change anything... ever!" -- Mr. Monk
>
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--
Fernando Cabral
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