Show matched lines for a given regex at Linux

Question

I use that notation at my log files:

[something-something]

somethings are necessary and variable length characters and numbers. My special log file includes line that has characters explained above. I.e.

Line 1: DEBUG [AS-kmksdf] Error occured!
Line 2: something somethingg..

I want to get Line 1. My log files grows dynamically and I show it at my screen with:

tail -f log.txt

How can I show just that special lines?

I know that I will use grep and my regex will be sth. like

[[A-Z]*-[A-Z]*]

how to combine them with that command. This doesn't work:

tail -f log.txt | grep [[A-Z]*-[A-Z]*]

I use cygwin with windows7 and can run some linux commands on my cmd.exe.

Answer 1

There are a couple of problems with your regular expression:

• First of all, I'd advice you to always use the -P switch, which enables Perl Compatible Regular Expressions.

  By default, grep uses Basic regular expression, which aren't used widely and suffer from inconsistent espacing.

• As is [A-Z], brackets have special meaning in regular expressions. To use a literal bracket in PCREs, escape it with a backslash: \[ or \]

• Regular expressions are case sensitive, unless you set the -i switch.

• * means any number of occurrences, including (0). You might want to use + instead (any positive number of occurrences).

• Always surround your regular expression with double quotes, or the bash will expand characters.

As a result, the following command will work:

tail -f log.txt | grep -Pi "\[[A-Z]+-[A-Z]+\]"

Answer 2

tail -f log.txt |grep -E '\[[a-zA-Z]*-[a-zA-Z]*\]'

You can use Regexpal to test your regex expressions. Also you should use -E on grep. It is not mandatory, and not necessary in this case, but allow you to use extended regex expressions.