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Comments on How to delete contents of a specific field, if it matches a pattern and there is nothing else in the field
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How to delete contents of a specific field, if it matches a pattern and there is nothing else in the field
How do I delete contents of a specific field, if it matches a pattern, and there is nothing else in the field? I have a several GB tsv file, and I am interested in a specific field (72). If it contains hyphens, and only hyphens, then I want the hyphens deleted, leaving a blank field. I am using Ubuntu 20.04, with GNU awk v5. I've tried something like this:
awk 'BEGIN{FS=OFS="\t"}{gsub(/-/,"",$72)}1' file.tsv
But that also deletes the hyphens if there are other characters in the field too, which I do not want. E.g.
blah------
becomes
blah
but I want to leave it unchanged; but change
---------------
to nothing.
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The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| LVx0 | (no comment) | Apr 11, 2022 at 07:56 |
The awk gsub function takes as its first argument a regular expression indicating the substring to be replaced, and replaces a matching substring with the value of the second argument, which is the replacement string.
Since your regular expression, /-/, is not anchored, it will match anywhere in the value that is being matched against. That's why it matches the hyphens that are contained as a part of the value. Here's an example run to illustrate this:
$ (printf '%s\n' 'blah---blah' '---blah---' 'blah------' '---------------') \
> | awk '{gsub(/-/,"",$1)}1'
blahblah
blah
blah
$
(note the empty line)
To get the behavior you are after, you need to anchor the regular expression at both the beginning and the end of the string, and provide a regular expression that matches against that whole string. Since you want to do a replacement only of strings that consist solely of hyphens, the regular expression should instead be /^-+$/ where the ^ anchors at the beginning, the -+ specifies an unbounded but non-zero number of - characters (that's the +), and the $ anchors at the end.
Example:
$ (printf '%s\n' 'blah---blah' '---blah---' 'blah------' '---------------') \
> | awk '{gsub(/^-+$/,"",$1)}1'
blah---blah
---blah---
blah------
$
As you can see, the contents of a field that consists of something other than only hyphens is now preserved, but the all-hyphens field contents is replaced by the empty string.
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