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Add sequence search help.


<i>This text is extracted from the scan_for_matches README, available in full 
<a href="scan_for_matches_README.html">here</a>.</i>

<h2>Simple Patterns Built by Matching Ranges and Reverse Complements</h2>

    Let me first explain this simple pattern:
                p1=4...7 3...8 ~p1

    The pattern consists of three "pattern units" separated by spaces.
    The first pattern unit is


    which means "match 4 to 7 characters and call them p1".  The
    second pattern unit is


    which means "then match 3 to 8 characters".  The last pattern unit

    which means "match the reverse complement of p1".  The first
    reported hit is shown as

        cguaacc ggttaacc gguuacg 

    which states that characters 6 through 27 of sequence tst1 were
    matched.  "cguaac" matched the first pattern unit, "ggttaacc" the
    second, and "gguuacg" the third.  This is an example of a common
    type of pattern used to search for sections of DNA or RNA that
    would fold into a hairpin loop.

<h2>Searching Both Strands</h2>

    Now for a short aside: scan_for_matches only searched the
    sequences in the input file; it did not search the opposite
    strand.  With a pattern of the sort we just used, there is not
    need o search the opposite strand.  However, it is normally the
    case that you will wish to search both the sequence and the
    opposite strand (i.e., the reverse complement of the sequence).
    To do that, you would just use the "-c" command line.  For example,

        scan_for_matches -c pat_file < test_dna_input

    Hits on the opposite strand will show a beginning location greater
    than te end location of the match.

<h2>Defining Pairing Rules and Allowing Mismatches, Insertions, and Deletions</h2>

    Let us stop now and ask "What additional features would one need to
    really find the kinds of loop structures that characterize tRNAs,
    rRNAs, and so forth?"  I can immediately think of two:

<li>       you will need to be able to allow non-standard pairings
           (those other than G-C and A-U), and

<li> you will need to be able to tolerate some number of
           mismatches and bulges.
    Let me first show you how to handle non-standard "rules for
    pairing in reverse complements".  Consider the following pattern,
    which I show as two line (you may use as many lines as you like in
    forming a pattern, although you can only break a pattern at points
    where space would be legal):

            p1=2...3 0...4 p2=2...5 1...5 r1~p2 0...4 ~p1        

    The first "pattern unit" does not actually match anything; rather,
    it defines a "pairing rule" in which standard pairings are
    allowed, as well as G-A and A-G (in case you wondered, Us and Ts
    and upper and lower case can be used interchangably; for example
    r1={AT,UA,gc,cg} could be used to define the "standard rule" for
    pairings).  The second line consists of six pattern units which
    may be interpreted as follows:

            p1=2...3     match 2 or 3 characters (call it p1)
            0...4        match 0 to 4 characters
            p2=2...5     match 2 to 5 characters (call it p2)
            1...5        match 1 to 5 characters
            r1~p2        match the reverse complement of p2,
                            allowing G-A and A-G pairs
            0...4        match 0 to 4 characters        
            ~p1          match the reverse complement of p1
                            allowing only G-C, C-G, A-T, and T-A pairs

    Thus, r1~p2 means "match the reverse complement of p2 using rule r1".
    Now let us consider the issue of tolerating mismatches and bulges.
    You may add a "qualifier" to the pattern unit that gives the
    tolerable number of "mismatches, deletions, and insertions".
                p1=10...10 3...8 ~p1[1,2,1]

    means that the third pattern unit must match 10 characters,
    allowing one "mismatch" (a pairing other than G-C, C-G, A-T, or
    T-A), two deletions (a deletion is a character that occurs in p1,
    but has been "deleted" from the string matched by ~p1), and one
    insertion (an "insertion" is a character that occurs in the string
    matched by ~p1, but not for which no corresponding character
    occurs in p1).  In this case, the pattern would match


    which is, you must admit, a fairly weak loop.  It is common to
    allow mismatches, but you will find yourself using insertions and
    deletions much more rarely.  In any event, you should note that
    allowing mismatches, insertions, and deletions does force the
    program to try many additional possible pairings, so it does slow
    things down a bit.

<h2>How Patterns Are Matched</h2>

    Now is as good a time as any to discuss the basic flow of control
    when matching patterns.  Recall that a "pattern" is a sequence of
    "pattern units".  Suppose that the pattern units were

        u1 u2 u3 u4 ... un

    The scan of a sequence S begins by setting the current position
    to 1.  Then, an attempt is made to match u1 starting at the
    current position.  Each attempt to match a pattern unit can
    succeed or fail.  If it succeeds, then an attempt is made to match
    the next unit.  If it fails, then an attempt is made to find an
    alternative match for the immediately preceding pattern unit.  If
    this succeeds, then we proceed forward again to the next unit.  If
    it fails we go back to the preceding unit.  This process is called
    "backtracking".  If there are no previous units, then the current
    position is incremented by one, and everything starts again.  This
    proceeds until either the current position goes past the end of
    the sequence or all of the pattern units succeed.  On success,
    scan_for_matches reports the "hit", the current position is set
    just past the hit, and an attempt is made to find another hit.
    If you wish to limit the scan to simply finding a maximum of, say,
    10 hits, you can use the -n option (-n 10 would set the limit to
    10 reported hits).  For example,

        scan_for_matches -c -n 1 pat_file < test_dna_input

    would search for just the first hit (and would stop searching the
    current sequences or any that follow in the input file).

<h2>Searching for repeats:</h2>

    In the last section, I discussed almost all of the details
    required to allow you to look for repeats.  Consider the following
    set of patterns:

        p1=6...6 3...8 p1   (find exact 6 character repeat separated
                             by to 8 characters)

        p1=6...6 3..8 p1[1,0,0]   (allow one mismatch)

        p1=3...3 p1[1,0,0] p1[1,0,0] p1[1,0,0]  
                            (match 12 characters that are the remains
                             of a 3-character sequence occurring 4 times)
        p1=4...8 0...3 p2=6...8 p1 0...3 p2
                            (This would match things like
                                ATCT G TCTTT ATCT TG TCTTT

<h2>Searching for particular sequences:</h2>

    Occasionally, one wishes to match a specific, known sequence.
    In such a case, you can just give the sequence (along with an
    optional statement of the allowable mismatches, insertions, and
    deletions).  Thus,

        p1=6...8 GAGA ~p1    (match a hairpin with GAGA as the loop)
        RRRRYYYY             (match 4 purines followed by 4 pyrimidines)
        TATAA[1,0,0]         (match TATAA, allowing 1 mismatch)

<h2>Matches against a "weight matrix":</h2>

    I will conclude my examples of the types of pattern units
    available for matching against nucleotide sequences by discussing a
    crude implemetation of matching using a "weight matrix".  While I
    am less than overwhelmed with the syntax that I chose, I think that
    the reader should be aware that I was thinking of generating
    patterns containing such pattern units automatically from
    alignments (and did not really plan on typing such things in by
    hand very often).  Anyway, suppose that you wanted to match a
    sequence of eight characters.  The "consensus" of these eight
    characters is GRCACCGS, but the actual "frequencies of occurrence"
    are given in the matrix below.  Thus, the first character is an A
    16% the time and a G 84% of the time.  The second is an A 57% of
    the time, a C 10% of the time, a G 29% of the time, and a T 4% of
    the time.  

             C1     C2    C3    C4   C5    C6    C7    C8
       A     16     57     0    95    0    18     0     0

       C      0     10    80     0  100    60     0    50

       G     84     29     0     0    0    20   100    50

       T      0      4    20     5    0     2     0     0   

    One could use the following pattern unit to search for inexact
    matches related to such a "weight matrix":

         (0,100,0,0),(18,60,20,2),(0,0,100,0),(0,50,50,0)} > 450

    This pattern unit will attempt to match exactly eight characters.
    For each character in the sequence, the entry in the corresponding
    tuple is added to an accumulated sum.  If the sum is greater than
    450, the match succeeds; else it fails.

    Recently, this feature was upgraded to allow ranges.  Thus,

  600 >  {(16,0,84,0),(57,10,29,4),(0,80,0,20),(95,0,0,5),
         (0,100,0,0),(18,60,20,2),(0,0,100,0),(0,50,50,0)} > 450

    will work, as well.

<h2>Allowing Alternatives:</h2>

    Very occasionally, you may wish to allow alternative pattern units
    (i.e., "match either A or B").  You can do this using something

                ( GAGA | GCGCA)

    which says "match either GAGA or GCGCA".  You may take
    alternatives of a list of pattern units, for example

        (p1=3...3 3...8 ~p1 | p1=5...5 4...4 ~p1 GGG)

    would match one of two sequences of pattern units.  There is one
    clumsy aspect of the syntax: to match a list of alternatives, you
    need to fully the request.  Thus,

        (GAGA | (GCGCA | TTCGA))

    would be needed to try the three alternatives.

</h2>One Minor Extension</h2>

    Sometimes a pattern will contain a sequence of distinct ranges,
    and you might wish to limit the sum of the lengths of the matched
    subsequences.   For example, suppose that you basically wanted to
    match something like

    ARRYYTT p1=0...5 GCA[1,0,0] p2=1...6 ~p1 4...8 ~p2 p3=4...10 CCT

    but that the sum of the lengths of p1, p2, and p3 must not exceed
    eight characters.  To do this, you could add 

        length(p1+p2+p3) < 9

    as the last pattern unit.  It will just succeed or fail (but does
    not actually match any characters in the sequence).

<h2>Matching Protein Sequences</h2>

    Suppose that the input file contains protein sequences.  In this
    case, you must invoke scan_for_matches with the "-p" option.  You
    cannot use aspects of the language that relate directly to
    nucleotide sequences (e.g., the -c command line option or pattern
    constructs referring to the reverse complement of a previously
    matched unit).  
    You also have two additional constructs that allow you to match
    either "one of a set of amino acids" or "any amino acid other than
    those a given set".  For example,

        p1=0...4 any(HQD) 1...3 notany(HK) p1

    would successfully match a string like

           YWV D AA C YWV

<h2>Using the show_hits Utility</h2>

    When viewing a large set of complex matches, you might find it
    convenient to post-process the scan_for_matches output to get a
    more readable version.  We provide a simple post-processor called
    "show_hits".  To see its effect, just pipe the output of a
    scan_for_matches into show_hits:

     Normal Output:

        clone% scan_for_matches -c pat_file < tmp
        gtacguaacc  ggttaac cgguuacgtac 
        gtacgtaacc  ggttaac cggttacgtac 
        gtacguaacc g gttaactt cgguuacgtac 
        gtacgtaacc g aagttaac cggttacgtac 

     Piped Through show_hits:
        clone% scan_for_matches -c pat_file < tmp | show_hits
        tst1:[1,28]:  gtacguaacc   ggttaac  cgguuacgtac
        tst1:[28,1]:  gtacgtaacc   ggttaac  cggttacgtac
        tst3:[3,32]:  gtacguaacc g gttaactt cgguuacgtac
        tst3:[32,3]:  gtacgtaacc g aagttaac cggttacgtac

    Optionally, you can specify which of the "fields" in the matches
    you wish to sort on, and show_hits will sort them.  The field
    numbers start with 0.  So, you might get something like

        clone% scan_for_matches -c pat_file < tmp | show_hits 2 1
        tst3:[32,3]:  gtacgtaacc g aagttaac cggttacgtac
        tst1:[1,28]:  gtacguaacc   ggttaac  cgguuacgtac
        tst1:[28,1]:  gtacgtaacc   ggttaac  cggttacgtac
        tst3:[3,32]:  gtacguaacc g gttaactt cgguuacgtac

    In this case, the hits have been sorted on fields 2 and 1 (that is,
    the third and second matched subfields).
    show_hits is just one possible little post-processor, and you
    might well wish to write a customized one for yourself.

<h2>Reducing the Cost of a Search</h2>

    The scan_for_matches utility uses a fairly simple search, and may
    consume large amounts of CPU time for complex patterns.  Someday,
    I may decide to optimize the code.  However, until then, let me
    mention one useful technique.  
    When you have a complex pattern that includes a number of varying
    ranges, imprecise matches, and so forth, it is useful to
    "pipeline" matches.  That is, form a simpler pattern that can be
    used to scan through a large database extracting sections that
    might be matched by the more complex pattern.  Let me illustrate
    with a short example.  Suppose that you really wished to match the

    p1=3...5 0...8 ~p1[1,1,0] p2=6...7 3...6 AGC 3...5 RYGC ~p2[1,0,0]

    In this case, the pattern units AGC 3...5 RYGC can be used to rapidly
    constrain the overall search.  You can preprocess the overall
    database using the pattern:

          31...31 AGC 3...5 RYGC 7...7

    Put the complex pattern in pat_file1 and the simpler pattern in
    pat_file2.  Then use,

	scan_for_matches -c pat_file2 < nucleotide_database |
        scan_for_matches pat_file1

    The output will show things like

    matches pieces

    Then, the actual section of the sequence that was matched can be
    easily computed as [233,278] (remember, the positions start from
    1, not 0).
    Let me finally add, you should do a few short experiments to see
    whether or not such pipelining actually improves performance -- it
    is not always obvious where the time is going, and I have
    sometimes found that the added complexity of pipelining actually
    slowed things up.  It gets its best improvements when there are
    exact matches of more than just a few characters that can be
    rapidly used to eliminate large sections of the database.


Feb 9, 1995:   the pattern units ^ and $ now work as in normal regular
	       expressions.  That is

			TTF $

	       matches only TTF at the end of the string and 

		        ^ TTF 

	       matches only an initial TTF

               The pattern unit 


	       matches the reverse of the string named p1.  That is,
	       if p1 matched GCAT, then <p1 would match TACG.  Thus,

		   p1=6...6 <p1

               matches a real palindrome (not the biologically common
	       meaning of "reverse complement")

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