grep "^[[:space:]]*clientPort[^[:alpha:]]" zoo.cfg
This is a grep pattern to extract the line that contains the port number from the configuration file.
1) Normally in a regex, I use square brackets to match any one of the enclosing characters. Here I am seeing :alpha:. Is it same as [a-zA-Z]?
2) Why there are double square parentheses in the regex?
3) [^[:alpha:]], What does the carat symbol between square brackets mean?
The configuration file contains the line,
clientPort=2181
1 Answer 1
Regular expression allow you to use Bracket Expression to represent the set of collating elements. The syntax for Bracket Expression is [...], where you can place in ... any of collating elements, collating symbols, equivalence classes, character classes, or range expressions.
The one you use alpha is a character class name, made by place class name between [: and :]. So, you have used the character class expression [:alpha:] in between Bracket Expression [[:alpha:]].
The character class expression is not the same as range expression [a-zA-Z] in some locales. Here's the best example you can see:
$ LC_ALL=en_US.utf8 bash -c 'case b in [A-Z]) echo yes; esac'
yes
while using character class gave you nothing:
$ LC_ALL=en_US.utf8 bash -c 'case b in [[:upper:]]) echo yes; esac'
The caret ^, if place at the beginning of Bracket Expression will negate the match of expression. [^[:alpha:]] will match any characters, which don't belong to [:alpha:] characters class:
$ case 1 in [^[:alpha:]]) echo yes;; esac
yes