| Re: stregex - lookaround operators? [message #56358 is a reply to message #56348] |
Thu, 18 October 2007 05:56   |
Spon
Messages: 178
Registered: September 2007
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Senior Member |
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On Oct 17, 5:35 pm, rtow...@gmail.com wrote:
> Does stregex support the lookahead and lookbehind operators? I'm
> guessing it doesn't, which is a real bummer. Specifically I am trying
> to use the lookahead operator to extract data from an XML like string:
> '[^>]*(?=</rid>)' which should extract all characters before the </
> rid> tag up to ">". But I get an error:
>
> STREGEX: Error processing regular expression: [^>]*(?=</rid>)
> repetition-operator operand invalid
>
> I think the expression is valid. Has anyone used the lookaround
> operators in IDL?
>
> -r
This is the best I could do. Not elegant, probably not efficient, but
it throws up a few questions of its own:
*******
FUNCTION STREGEXTEST, StrExpr
IF N_PARAMS() EQ 0 THEN $
StrExpr = ['<rid> Stuff in here </rid>', $
'<rid> All of this stuff in here </rid>', $
'<html> Not this stuff </html>', 'frog', $
'>.<', '<grid> Not this either </grid>', $
'</html> <rid> This stuff too </rid>', $
'<rid> Even this</rid> <foo>!</foo>', $
'The acid test </rid>' ]
; StrExprTemp = StrExpr
RegStr = '>[^>]*(</rid>)'
Streg = STREGEX (StrExpr, RegStr, $
LENGTH = Length)
ExtrStreg = STREGEX (StrExpr, RegStr, /EXTR)
PRINT, Streg
PRINT, ExtrStreg
Index = WHERE (Streg GE 0, Count)
IF Count EQ 0 THEN RETURN, ''
Streg = Streg [Index]
Length = Length [Index]
PosExp = StrExpr [Index]
Streg += 1 ; Manually move your location
Length -= 7 ; and length pointers to
; exclude the Lookarounds
; ; This doesn't work for string arrays,
; ; even if Streg and Length have the
; ; same dimensions:
; Strings = STRMID (PosExp, Streg, Length)
NS = N_ELEMENTS (Streg)
Strings = STRARR (NS)
FOR i = 0L, NS - 1 DO $
Strings [i] = STRMID (PosExp [i], $
Streg [i], Length [i] )
HELP, Strings
PRINT, Strings
RETURN, Strings
END
********
As you can see, I had foolishly assumed that if you call STRMID with
three arrays of equal dimensions, IDL would somehow know to
'vectorise' the calculation, but I had to pump it through a FOR loop
instead. Can someone show me a way around this?
The other commented-out line is this:
; StrExprTemp = StrExpr
Which I was using because I thought STREGEX (or possibly STRMID) was
changing my input function (I've written functions of my own that
ended up doing this, and watched in dismay as my nice 4-dimensional
array was decimated to a tiny subset, when all I was doing was trying
to was extract the subset from the data, leaving the original intact.
It's easy not to do a second time, but quite frustrating the first
time you do it!)
I seemed to be getting a large array of blank strings after using
STREGEX and them STRMID on the input array (the resulting array was of
expected dimensions, just empty, contrary to my expectations.) I can't
seem to replicate this now, so chances are it was some silly error on
my part, but I thought I'd ask if anyone else had come across this?
Thanks,
Chris
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