grep '^[^1]' h.txt (what does the " ' " character indicate

babu198649 · 12-27-2007, 12:34 AM

hi
i have searched many sites for regular expressions. they all ignore to tell about one character. what does the " ' " (single quotes) signify.

such as in the following example .

grep '^[^1]' h.txt

does they have special meaning or it is a syntax( for linux commands).

gerbenny · 12-27-2007, 07:32 AM

The single quote is an alternate quote character. More of a strict quote. Can be used to encase literal double-quotes, instead of escaping them.

eg
sed "s/\(abcd\"abcd\)/\1/g"
Out - abcd"abcd

sed 's/\(abcd"abcd\)/\1/g'
Out - abcd"abcd

rupertwh · 12-27-2007, 07:51 AM

Quote:

Originally Posted by babu198649

grep '^[^1]' h.txt

Those quotes are interpreted by the shell, not grep. grep never sees those quotes.

babu198649 · 12-28-2007, 04:50 AM

thank u ..........

archtoad6 · 12-29-2007, 11:32 AM

That sed command can get even cleaner:

Code:

sed -r 's/(abcd"abcd)/\1/g'

and prettier:

Code:

sed -r 's,(abcd"abcd),\1,g'

As for the single quotes, they are telling the shell not to interpret anything found between them, & to pass the enclosed argument(s) to grep, sed, or whatever.

If you want to pass the value (as opposed to its literal name) of a shell to a program, then use the double quotes.

It is a good habit, rarely a mistake, to enclose the regex passed to grep in single quotes.