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Re: Regular Expressions [message #24195 is a reply to message #24188]

Thu, 15 March 2001 19:24

Wayne Landsman
Messages: 117
Registered: January 1997

Senior Member

JD Smith wrote:

> IDL> print, stregex(st,'(^|[^l])l($|[^l])')
>
> which means "a character that is not 'l', or the beginning of the
> string, followed by an 'l', followed by a character that is not 'l', or
> the end of the string". Aren't you glad Ken Thompson didn't decide
> originally to develop regexps in english?
>
> This will also work on
>
> IDL> st = "let's all go the the movies"

Thanks. But I now realize that my original formulation was not quite
correct, since the above expression (usually!) returns the position of the
character *before* the 'l', so to get the position of the first single 'l'
one has to add 1

IDL> l_position = stregex(st,'(^|[^l])l($|[^l])') + 1

Unfortunately, if 'l' is the first character, then you *don't* want to add
the 1. (The expression stregex(st,'(^|[^l])l($|[^l])') returns a value of
0 for both st ='long days' and st ='slow nights'. )
One solution is to forget about the beginning of string anchor and just
concatenate a blank to the beginning to the string

IDL> l_position = stregex(' ' + st,'[^l]l($|[^l])')

--Wayne

P.S. The real-life problem I am working on deals not with 'l' but with
apostrophes. I am trying to speed up the processing of FITS header
values, where is a string is delineated by non-repeating apostrophes, and a
possessive is indicated by a double apostrophe.

VALUE = 'This is Wayne''s FITS value' / Example string field

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[Message index]

		regular expressions By: Michael Werger on Fri, 04 June 1999 00:00
		Re: Regular expressions By: Brian Jackel on Fri, 10 November 2000 00:00
		Re: Regular expressions By: Martin Schultz on Fri, 10 November 2000 00:00
		Re: Regular expressions By: Liam E. Gumley on Fri, 10 November 2000 00:00
		Re: Regular Expressions By: John-David T. Smith on Thu, 15 March 2001 16:09
		Re: Regular Expressions By: Wayne Landsman on Thu, 15 March 2001 19:24
		Re: Regular Expressions By: John-David T. Smith on Fri, 16 March 2001 09:20
		Re: Regular Expressions By: Pavel A. Romashkin on Mon, 19 March 2001 13:52
		Re: Regular Expressions By: Craig Markwardt on Tue, 20 March 2001 15:23
		Re: Regular Expressions By: John-David T. Smith on Tue, 20 March 2001 15:28
		Re: Regular Expressions By: Mark Hadfield on Tue, 20 March 2001 14:48
		Re: Regular Expressions By: Wayne Landsman on Tue, 20 March 2001 12:14
		Re: Regular Expressions By: Martin Schultz on Tue, 20 March 2001 04:39

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