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Suppose I have a foo.txt file with the following content:
[2010-11-13 12:00:02,656]
[2010-11-13 12:00:02,701]
[2010-11-13 12:00:02,902]
When I ack for the date portion with the following, it works:
ack "(?P<foo>\d{4}-\d{2}-\d{2})" foo.txt --output "\$1"
2010-11-13
2010-11-13
2010-11-13
But when I try to use --output with the named group "foo", I cannot get it to work:
ack "(?P<foo>\d{4}-\d{2}-\d{2})" foo.txt --output "(?P=foo)"
(?=foo)
(?=foo)
(?=foo)
Any help is greatly appreciate it. Thanks so much.
365
If your regular expression has named capturing groups, then you should use named backreferences to them in the replacement text. The regex (?' name'group) has one group called “name”. You can reference this group with ${name} in the JGsoft applications, Delphi, .
What is Group in Regex? A group is a part of a regex pattern enclosed in parentheses () metacharacter. We create a group by placing the regex pattern inside the set of parentheses ( and ) . For example, the regular expression (cat) creates a single group containing the letters 'c', 'a', and 't'.
tl;dr non-capturing groups, as the name suggests are the parts of the regex that you do not want to be included in the match and ?: is a way to define a group as being non-capturing. Let's say you have an email address [email protected] . The following regex will create two groups, the id part and @example.com part.
ack "(?P<foo>\d{4}-\d{2}-\d{2})" foo.txt --output "\$+{foo}"
(?P=foo) only works inside the regular expression (it's the named equivalent of \1). $+{foo} is the named equivalent of $1.
Also, with most shells, single quotes will help you avoid extra backslashes:
ack '(?P<foo>\d{4}-\d{2}-\d{2})' foo.txt --output '$+{foo}'
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