Query to select a string between two known strings

declare @s varchar(100)
set @s='here is some other text/a/b/c/d-this text has to be selected-/q/w/e/rand here is yet some more text'
select substring(@s,charindex('/a/b/c/d',@s)+len('/a/b/c/d'),charindex('/q/w/e/r',@s)-charindex('/a/b/c/d',@s)-len('/a/b/c/d'))

You could of course do it with Regular Expressions as well, but then you'd have to install a CLR to support the Regex functions.

    /* lets assume that SOME strings ARE NULL AND SOME strings
       don't even have the delimiters. Some only have the first, 
       some only have the second and some don't have either. 
       Unfair? Nah, just you wait until this gets into production 
       If either are missing we assume the beginning of the string */
    -- CHARINDEX('-/q/w/e/r',MangledString+'-/q/w/e/r')


    SELECT LEFT(topped, CHARINDEX('-/q/w/e/r',topped+'-/q/w/e/r')-1)
    FROM
      (SELECT 
             RIGHT(mangledString,CHARINDEX(REVERSE('/a/b/c/d-')
                        ,REVERSE('/a/b/c/d-'+MangledString))-1) 
      FROM (VALUES
    ('i know that /a/b/c/d- and -/q/w/e/r are constants'),
    ('i know that there might be just one delimiter -/q/w/e/r '),
    ('/a/b/c/d-i know that there might be just the first one '),
    (NULL),
    ('He who is ridden by a conscience /a/b/c/d-Middle-aged life is merry, and I love to lead it,-/q/w/e/r Because everybody would love to have a baby around who was no  responsibility and lots of fun, '), 
    ('Worries about a lot of nonscience;/a/b/c/d-But there comes a day when your eyes are all right but your arm long enough to hold the telephone book where you can read it, -/q/w/e/rBut I can think of no one but a mooncalf or a gaby'),
    ('He without benefit of scruples/a/b/c/d-And your friends get jocular, so you go to the oculist,-/q/w/e/r Who would trust their own child to raise a baby. '), 
    ('His fun and income soon quadruples/a/b/c/d-And of all your friends he is the joculist,-/q/w/e/rSo you have to personally superintend your grandchild from diapers  to pants and from bottle to spoon,'), 
    ('People live forever in Jacksonville and St. Petersburg and Tampa,  /a/b/c/d-So over his facetiousness let us skim,-/q/w/e/r'), 
    (' But you don''t have to live forever to become a grampa. /a/b/c/d-Only noting that he has been waiting for you ever since you said Good evening to his grandfather clock under the impression that it was him,-/q/w/e/rBecause you know that your own child hasn''t sense enough to come  in out of a typhoon.  '), 
    (' The entrance requirements for grampahood are comparatively mild/a/b/c/d-And you look at his chart and it says SHRDLU QWERTYOP, and you say Well, why SHRDNTLU QWERTYOP? and he says one set of glasses won''t do.-/q/w/e/rYou don''t have to live forever to become a grampa, but if you do  want to live forever,  '), 
    ('You only have to live until your child has a child./a/b/c/d-You need two. -/q/w/e/rDon''t try to be clever; '),
    ('From that point on you start looking both ways over your shoulder/a/b/c/d-One for reading Erle Stanley Gardner''s Perry Mason and Keats''s "Endymion" with,-/q/w/e/r If you wish to reach the end of the trail with an uncut throat'), 
    (' Because sometimes you feel thirty years younger and sometimes  thirty years older.  /a/b/c/d-And the other for walking around without saying Hello to strange wymion with.-/q/w/e/r  Don''t go around saying Quote I don''t mind being a grampa but I  hate being married to a gramma Unquote. '),
    ('Now you begin to realize who it was that reached the height of  imbecility,/a/b/c/d-So you spend your time taking off your seeing glasses to put on your reading glasses, and then remembering that your reading glasses are upstairs or in the car, -/q/w/e/r-/q/w/e/r-/q/w/e/r-/q/w/e/r-/q/w/e/r-/q/w/e/r-/q/w/e/r'),
    (' It was whoever said that grandparents have all the fun and none of  the responsibility./a/b/c/d-And then you can''t find your seeing glasses again because without them on you can''t see where they are. -/q/w/e/r-/q/w/e/rcv e  ewf  we w wer wer wer wer wer we wer we '),
    ('This is the most enticing spiderwebs of a tarradiddle ever spun, /a/b/c/d-Enough of such misshaps, they would try the patience of an ox, -/q/w/e/r

    '),
    ('I prefer to forget both pairs of glasses and pass my declining years saluting strange women and grandfather clocks.'))
      F(MangledString))
    g(topped)

    /*
     and 
    i know that there might be just one delimiter 
    i know that there might be just the first one 
    NULL
    Middle-aged life is merry, and I love to lead it,
    But there comes a day when your eyes are all right but your arm long enough to hold the telephone book where you can read it, 
    And your friends get jocular, so you go to the oculist,
    And of all your friends he is the joculist,
    So over his facetiousness let us skim,
    Only noting that he has been waiting for you ever since you said Good evening to his grandfather clock under the impression that it was him,
    And you look at his chart and it says SHRDLU QWERTYOP, and you say Well, why SHRDNTLU QWERTYOP? and he says one set of glasses won't do.
    You need two. 
    One for reading Erle Stanley Gardner's Perry Mason and Keats's "Endymion" with,
    And the other for walking around without saying Hello to strange wymion with.
    So you spend your time taking off your seeing glasses to put on your reading glasses, and then remembering that your reading glasses are upstairs or in the car, 
    And then you can't find your seeing glasses again because without them on you can't see where they are. 
    Enough of such misshaps, they would try the patience of an ox, 
    I prefer to forget both pairs of glasses and pass my declining years saluting strange women and grandfather clocks.

    */

As a variation to the answer from @magnus ahlkvist ...

DECLARE @s VARCHAR(100)
DECLARE @start INT
DECLARE @end INT
DECLARE @FirstPatn CHAR(8) 
DECLARE @SecondPatn CHAR(8)

SET @s = 'here is some other text/a/b/c/d-this text has to be selected-/q/w/e/rand here is yet some more text'
SET @FirstPatn = '/a/b/c/d'
SET @SecondPatn = '/q/w/e/r'

SELECT  @start = PATINDEX('%' + @FirstPatn + '%', @s) ,
        @end = PATINDEX('%' + @secondPatn + '%', @s)

SELECT  SUBSTRING(@s, @start + LEN(@firstpatn),
                  ( @end - @start ) - LEN(@secondpatn))

@Jonathan Allen:

Unless I'm missing something, shouldn't you be subtracting LEN(@firstpatn) from the second part of that equation rather than LEN(@secondpatn) when you're selecting the substring?

e.g.

... SELECT SUBSTRING(@s, @start + LEN(@firstpatn), ( @end - @start ) - LEN(@firstpatn))

It appears that it worked in the example you provided because @FirstPatn and @SecondPatn happened to be the same length.

Other than that, your example made my life a lot easier, thanks for the answer!