Difference between revisions of "Regular expressions"
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Posts like this birhgten up my day. Thanks for taking the time. | Posts like this birhgten up my day. Thanks for taking the time. | ||
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== Match one or more times == | == Match one or more times == | ||
Revision as of 14:32, 23 May 2011
Regular expressions allow you to formulate patterns to search for. Here's an example: It is easy to search for the string "Sep" in a file, you do it with
grep "Sep" file
This gives you all lines containing the string "Sep". But what do you do if you only want lines starting with "Sep", for example, to read all lines in your syslog regarding september? Then you need regular expressions. It works like this:
grep -E "^Sep" /var/log/messages
gives you all entries for september in your syslog. And there is much more you can do with regular expressions.
Contents
Escaping
The characters ^ and \ are seen as control-characters. ^ means "at the beginning of a line". With a backslash, you can escape these control-characters, meaning they act as body-characters again:
grep "^hallo" file
finds all occurrences of "hallo" at the beginning of a line in file.
grep "\^hallo"
finds all occurrences of "^hallo" in a file
grep "\\^hallo"
finds all occurrences of "\^hallo" in a file
grep "\\\\^hallo"
finds all occurrences of "\\^hallo" in a file And so on...
Write regular expressions
For "finding a pattern defined by a regular expression", we speak of "matching".
That's way more clever than I was epxecitng. Thanks!
The end of a line
grep "hallo$" file
prints all occurrences of "hallo" at the end of a line in file.
Find string1 OR string2
grep -E "Sep|Aug" file
prints all lines from file that contain "Sep" or "Aug".
I'm out of league here. Too much brain power on dipasly!
Match a range of characters
grep -E "foo[1-9]" file
prints all lines from file that contain "foo1" or "foo2" till "foo9"
Posts like this birhgten up my day. Thanks for taking the time.
That's 2 clveer by half and 2x2 clever 4 me. Thanks!
Match one or more times
grep -E "L[i]+nux" file
Match if i is there at least once in file The + here is a quantifier. It means, that i occurs 1 or more times. It is also possible to accept 0 or more times if you replace the + by a *.
Read regular expressions
*
An asterisk is a quantifier saying "whatever number of".
grep -E "Li*nux" file Lnux Linux Liinux Liiinux
An asterisk is placed next to an atom that can be repeated in whatever number. In the above example, the atom is the i character, but it can also be a group of characters:
grep -E "ba(na)*" file ba bana banana bananana
^
The ^ character stands for
- the beginning of a line if it stands at the beginning of a branch
# grep ^foo barfoo foo foo
- "not" if it stands behind a bracket
# grep for[^e] foresee for each for each
- the ^ character if it is escaped
# grep "\^" adsf as^df as^df
Your ansewr was just what I needed. Its made my day!
