Using a perl compatible regex with GNU grep -P

grepregular expression

I'm using this regex (?<=\[')[^,]* on a file containing the following line disk = ['OVS/sdasd/asdasd/asdasd/something.img, w']

I want that to return OVS/sdasd/asdasd/asdasd/something.img

How do I use grep to make it work?

I've tried grep -P "(?<=\[')[^,]*" but it returns the whole line.

Best Answer

Add the -o switch so that grep only returns what matches the pattern you're grepping for:

$ grep -Po "(?<=\[')[^,]*" data.txt 
OVS/sdasd/asdasd/asdasd/something.img

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With normal regex, the characters (, | and ) need to be escaped. So you should use

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PCRE's are not literally perl; they use a stand-alone C library, libpcre. Sometimes "PCRE style" is also used to refer to, e.g., other languages which implement regexps using the same/similar patterns, even though they do not rely on libpcre (note grep does use the actual libpcre).

Strictly speaking, a regular expression doesn't really include modifiers,¹ it's just a pattern. Some regexp interfaces (such as perl's) allow for the use of modifiers along with the patterns; grep doesn't.

There are a few differences in terms of the pattern semantics too. Again, keep in mind it's not actually perl, it's something based on it.

^{1. Unless they're actually inside the pattern, which you can do with grep. E.g. grep -P "(?i)foobar" will be a case insensitive match.}

Best Answer

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