Circos > Documentation > Tutorials > Utilities > Filtering Links

Use the latest version of Circos and read Circos best practices—these list recent important changes and identify sources of common problems.

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10 — Helper Tools

4. Filtering Links

script location


script usage

> cd tools/filterlinks
> ./run
Processing data/dog.vs.human.subset.txt
Now look in data/dog.vs.human.subset.filtered.txt

The script sends a new links file to STDOUT and the report shown above to STDERR.

To get the full manpage, use -man.

> cd tools/filterlinks
> bin/filterlinks -man

Adjust the configuration file etc/filterlinks.conf to suit your needs.


This script is used to generate a new link file, composed of links that pass certain rules.

Although Circos supports rules in its configuration file, which you can use to alter data formatting on the fly, you may find that creating a new link file is more convenient.

For example, if you are using Circos in an interactive web page, you want to generate the image as quickly as possible. Since Circos' built-in rule engine is slower than this script, filterlinks is handy way to eliminated unwanted data upstream of Circos.

Another situation in which filterlinks is useful is to generate an input file to orderchr, which minimizes crossing of links by shuffling chromosomes around the image circle. Since the speed of orderchr proportional (in a nasty way) to the number of links, it's important to pass it as many links as you need to get the job done, but no more than that.

You filter on any property of the link with filterlinks:

  • link id
  • chromosome
  • start and end position
  • size
  • span (intersect with genomic region)
  • format options (color, thickness, z, etc)

and any property can be tested with one of these rules

  • regular expression (e.g. chr1 matches "x")
  • exact string match (e.g. chr1 is "hs1")
  • smaller and greater than (numerical or string order) (e.g. start < 10Mb)
  • integer span (e.g. link span intersects 10-20Mb)

Briefly (read the manpage below for all the details), the results of each rule test are all OR'ed together. Thus, it is sufficient for one rule to pass to pass the link.

Thus, if you have three rules, the link will be passed if

    RULE1 or RULE2 or RULE3

By assigning IDs to sets of rules, you can group rules together to build up a complex filter which includes AND. In the example below, rules 1-3 have been assigned a distinct ID (0) and rules 4-5 were given ID 1. These IDs control how rule results are combined: intra-ID rules are AND'ed and inter-ID rules are OR'ed.

  ( RULE1 and RULE2 and RULE3 ) OR ( RULE4 and RULE5 )
    _________________________        _______________
    ^ rule set 1 (e.g. ID=0)         ^ rule set 2 (e.g. ID=1)

filterlinks manpage


filterlinks - filter the link file based on link parameters


  filterlinks -links linkfile.txt [-nointer] [-nointra] [-debug]



A filter rules contains two parts: the link parameter which is tested and a list of acceptable conditions.

The two exceptions are the -nointer and -nointra flags. These can be used to filter out inter-chromosomal links (ends of link are on different chromosomes) and intra-chromosomal links (ends of link are on the same chromosome). These two rules are strict, meaning that if a link does not pass them, no other rules are tested and the link is immediately rejected.

Link Parameters

  link_param = condition1,condition2,...

Because each link has two ends, each link parameter may give rise to three distinct rules

  link_param   = condition1,condition2,...
  link_param_1 = condition1,condition2,...
  link_param_2 = condition1,condition2,...

which test, respectively, both ends with the condition (both ends must pass), the first end, and the second end. The first end of the link corresponds to the first line of the link line pair. For example, given the link ... link018136 cf12 9800000 9900000 link018136 hs6 37914056 37916509 ...

the first end is cf12:9800000-9900000 and the second end is hs6:37914056-37916509.

  • chr

    Applies the condition to the chromosome of the link.

      chr   = 1
      chr_2 = x
  • start, end, size

    Applies the condition to the start, end or size of the link. The link size is end-start+1.

      start = [?<]10000000
      end   = [?>]50000000
  • span

    Applies the condition to the span of the link and should be used with the ``s'' condition TYPE.

      span = [?i]1000-2000
  • id

    Applies the condition to the id of the link.

  • color, thickness, z, etc.

    Any condition that is not one of id, chr, start, end, size, span is assumed to be a link option and is applied to the option of the link. For example, options include color, thickness, and z.

      color = [?e]chr12
      z = [?>]10


A condition has the following format

  { [?TYPE {ID} {!} ] } CONDITION

where elements in { } are optional. Briefly, TYPE is used to indicate how the CONDITION text should be applied (e.g. regular expression, integer range, exact match, etc). The ID is used to combine rules so that their match status is AND'ed together to determine whether the link passes. The trailing ``!'' is used to negate the rule (i.e. for the link to pass, the rule must fail).

  • Default Condition is a Regular Expression

    If no optional elements in the condition are specified, it is treated as a regular expression. For example,

      LINK_PARAM = 12

    would apply the regular expression ``12'' to the link parameter. You can provide a list of conditions with ;; as a delimiter (you can adjust the delimiter in the configuration file).

      LINK_PARAM = 12;;x;;y

    which are interpreted as a series of regular expressions used to test the link parameter. The link will be passed if ANY condition matches (i.e. match results are OR'ed). If you want match results to be AND'ed (i.e. multiple rules must match for the link to pass, read below).

    The regular expression is case-insensitive.

  • Adjusting Condition Type

    The following conditions types are possible

    • r - regular expression (default)

        LINK_PARAM = 12
        LINK_PARAM = 12;;x;;y

      You can specify the type as a regular expression explicitly with [?r] but this is not necessary because that is the default.

        LINK_PARAM = [?r]12;;[?r]x;;[?r]y
    • s - integer span

      The syntax of the integer range is any string supported by Set::IntSpan.

        LINK_PARAM = [?s]1000-2000
        LINK_PARAM = [?s]1000-2000,3000-4000
        LINK_PARAM = [?s]1000-2000,3000-)
        LINK_PARAM = [?s](-1000,2000,3000-)
        LINK_PARAM = [?s](-1000,2000,3000-4000,5000-)
    • e - exact match

      The exact match is useful for matching chromosome names in cases where regular expressions might match other chromosomes (and you don't want to include anchors in your regular expression).

        LINK_PARAM = [?e]chr1
        LINK_PARAM = [?e]chr1;;[?e]chr2

      Note that the condition type must be prefixed to each individual condition, if a list of conditions is supplied.

      The exact match is not case-sensitive.

    • < - less than

      If the value is a number, numerical < is used, otherwise the values are compared based on asciibetic order (e.g. le).

        # LINK_PARAM must be less than 100
        LINK_PARAM = [?<]100

        # LINK_PARAM must be less (in the asciibetic sense) than chr20 (e.g. chr1, chr11, chr111, chr19, etc)
        LINK_PARAM = [?<]chr20
    • > - greater than

      Works just like the less than condition [?<].

    • mixing condition types

      You can have multiple condition types for a parameter. Remember that results of each condition will be OR'ed together.

        LINK_PARAM = 1,[?e]chr5,[?e]chr22

      The first condition is a regular expression (by default). The second and third conditions are exact text matches for chr5 and chr22. Thus, the LINK_PARAM will pass if (a) it contains a ``1'', or (b) it is ``chr5'' or (c) it is ``chr22''.

  • Negating a Condition

    In order to negate a condition, use ``!''. When ``!'' is used, the condition must fail for the result to be acceptable.

      # must not match regular expression "1"
      LINK_PARAM = [?r!]1

      # must not be "chr12"
      LINK_PARAM = [?e!]chr12

      # must not be within the range 1000-2000
      LINK_PARAM = [?i!]1000-2000

    In order to combine negated conditions with positive ones, you will need to group conditions so that their results are AND'ed.

  • Grouping Conditions

    So far, all condition results were evaluated with OR. In other words, if you had a list of conditions, the successful pass of any of the conditions resulted in the link being passed. This is useful if you want to accept multiple values

      # chr12 or chr14 
      LINK_PARAM = [?e]chr12;;[?e]chr14

    However, what if you wanted to match regular expression ``1'' but not chr14. Here's where the ID field comes in. By tagging multiple conditions with the same ID field the results of each of these conditions is AND'ed together to determine whether the link passes.

      # ID=0 
      # match regular expression "1" AND not be "chr14"
      LINK_PARAM = [?r0]1;;[?e0!]chr14


Below are some examples to get you started. Note the interplay between conditions with IDs and condition without IDs. The former collate conditions into AND'ed sets, which are then in turn OR'ed with other sets and with conditions without IDs.

Filtering by Chromosomes

To select links in which both ends match regular expression ``1''

  chr = 1

So simple. Now, to select links in with either ends matches regular expression ``1'',

  chr_1 = 1
  chr_2 = 1

The difference between these two cases is that in the first instance, since the link_parameter does not include a _1 or _2 suffix, the condition is applied to both ends of the link and both ends must pass. In the second case, each end is tested independently and the results are OR'ed together.

If you want links where the first chromosome matches x or the second matches y,

  chr_1 = x
  chr_2 = y

The test is (chr_1 match ``x'') OR (chr_2 match ``y''). Note, however, that this set of rules requires that the first chromosome match ``x'' OR the second chromosome match ``y''. It will fail if the first chromosome matches ``y'' and the second matches ``x''. To match both possibilities, you might try

  chr_1 = x;;y
  chr_2 = y;;x

In this case the test is (chr_1 match ``x'') OR (chr_1 match ``y'') OR (chr_2 match ``x'') OR (chr_2 match ``y'').

If you are looking for links between x and y chromosomes, then you require the results of each condition to be AND'ed. For this, use IDs

  chr_1 = [?r1]x
  chr_2 = [?r1]y

Both of these rules have ID=1 and are therefore grouped into a set. Match results within a set are AND'ed. Thus, the test is (chr_1 match ``x'') AND (chr_2 match ``y''). If you want to match the other order too,

  chr_1 = [?r1]x;;[?r2]y
  chr_2 = [?r1]y;;[?r2]x

In this example, there are two IDs. The rules with ID=0 match chr1 to ``x'' and chr2 to ``y'' and the rules with ID=1 match the converse (chr1 to ``y'' and chr2 to ``x'').

Now let's suppose we want links that are either cf1-hs6, cf14-hs7 or cfx-hsx. Here cf is a dog chromosome and hs is a human chromosome. The rule for this is

  chr_1 = [?e1]cf1;;[?e2]cf14;;[?e3]cfx
  chr_2 = [?e1]hs6;;[?e2]hs7;;[?e3]hsx

You can add additional conditions without IDs to accept more links. For example, if you also wanted to add any links for which chr_1 was cf9 or for which chr_2 matched ``3''

  chr_1 = [?e1]cf1;;[?e2]cf14;;[?e3]cfx;;[?e]cf9
  chr_2 = [?e1]hs6;;[?e2]hs7;;[?e3]hsx;;3

Remember that [?r]3 is the same as 3, since the default condition type is a regular expression.

You can take advantage of the ``!'' flag to negate rules to avoid chromosomes. For example, if you want links between cfx and any chromosome other than hsx

  chr_1 = [?e1]cfx
  chr_2 = [?e1!]hsx

and here the test is (chr_1 is cfx) AND (chr_2 is not hsx).

You can combine chr with chr_1/chr_2 rules

  chr   = 2
  chr_1 = [?e1]cfx
  chr_2 = [?e1!]hsx

to produce the test ( (chr_1 is cfx) AND (chr_2 is not hsx) ) OR ( chr_1 matches ``2'' AND chr_2 matches ``2'' ). Use ``chr'' as the parameter if you want to apply the same condition to both ends of th elink and chr_1 and chr_2 to apply different conditions.

Filtering by Position

To test link position, use the parameters ``start'', ``end'' and ``span''. Both ``start'' and ``end'' are ideal for testing with condition type < and >. To select links for which both ends start before 10,000,000

  start = [?<]1e7
  # or
  start = [?<]10000000

to add another OR'ed condition to pass links with start values beyond 100,000,000

  start = [?<]1e7;;[?>]1e8

A more complex test for the ``start'' and ``end'' values can be leveled by using the ``s'' condition type, which tests for membership within a span. This rule

  start = [?i]1e6-2e6,3e6-4e6

will pass links for which both ends are within 1-2Mb or 3-4Mb. Note that the ``,'' in this condition is part of the span and does not create a new condition. To have two conditions, use the ;; delimiter.

  start = [?i]1e6-2e6,3e6-4e6;;[?s]1e7-1.1e7,3e6-4e6

When using the ``span'' parameter, you should always use the ``s'' condition type. This will check whether the link span intersects the provided span.

  span = [?s]2e7-5e7

This will select all links whose spans (at both ends) intersect the coordinates 20-50Mb. To be more selective, use the _1 and _2 suffixes.

  span_1 = [?s1]2e7-5e7
  span_2 = [?s1]2e7-2.5e7

will select links joining 20-50Mb regions to 20-25Mb regions. An ID was required here to make the results AND'ed. To avoid certain regions, use the ``!'' flag

  span = [?s!](-1e7

will avoid all links within the first 10Mb.

Filtering by Link Options

Any link option such as ``color'', ``thickness'', or ``z'' can be tested in similar rules.

  # links with z value greater than 10
  z = [?>]10 

  # links with z value between 5 and 15
  z = [?s]5-15

Mixing Conditions and IDs

You can write fairly complex rules by combining different link parameter, rule types and IDs.

For example to apply the following filter

  between (hs1 and cf6) 
  within 75-80 Mb on hs1
  larger than 5kb on hs1


  larger than 500kb on hs1

use the following rules

  chr_1   = [?e1]cf6
  chr_2   = [?e1]hs1
  span_2  = [?s1]75e6-80e6
  size_2  = [?>1]5e3;;[?>]500e3




Martin Krzywinski


  Martin Krzywinski
  Genome Sciences Centre
  Vancouver BC Canada