steffen
2020-02-19 07:46:49
The problem with regular expressions in grep and other Unix tools is that they usually support one, two or three different kinds of regular expressions. These are:
- Basic regular expressions (BRE)
- Extended regular expressions (ERE or EREG)
- Perl compatible regular expressions (PCRE or PREG)
Your pattern is in PCRE syntax, therefore you need to identify your pattern as one (using -P). Note that I also removed the m between = and % (I don't know what that was supposed to do).
grep -Po "(?<=%s)(.*?)(?=\.com)" myfile.txt
With -o, you say you only want to print the matching part. My grep man page declares PCRE in grep as experimental so there probably might be cases where you'd get a segmentation fault or where the evaluation takes unusually much time.
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And the difference of the deleted
min the lookbehind condition .... ?(?<=m%s)!=(?<=%s)I actually don't know what that
mwas supposed to do.But you deleted it. Note that I totally agree wiht deleting it, because I do not see how a regex including it could ever match. But the fact that it is there is enough explanation for it not working and makes the whole question mostly off-topic as "not reproduceable/typo". Just mention it as necessarily deleted, even if it is only a typo.
Note, there is an alternative, using
\K. I thought that would allow to make a regex without PCRE support, but it seems I was wrong. It does work (tested on regex101.com), but is also PCRE, according to riptutorial.com/regex/topic/1338/match-reset---k I am not going to make a separate answer for this. Feel free to add this info to your answer, to make it more complete.@Yunnosch Yes, I know (see stackoverflow.com/a/52796549/845034). But I don't get the point here. There are always more solutions to problems. I guess, the OP just wanted to know, why is regex did not work.