Re: Question relating regexp
On Fri 17 Jan 2003 15:03:33 +0000(+0100), Robert Land wrote:
>
> To match a line that starts with a 1,
> has some digits, at least one space
> and a name that starts with a K we can write:
>
> grep '^1[0-9]\{1,\} \{1,\}K' phonelist.txt
> or use * and repeat [0-9] and space:
> grep '^1[0-9][0-9]* *K' phonelist.txt
> "
>
>
> =Why, in the first example, has the author
> prefaced the char 'K' with the one or more
> times multiplier? He only wants to find a
> name beginning with 'K'(!)
He/she hasn't. The "one or more times multiplier" applies to the preceding element (a space). Let's break this regular expression down:
^ matches start of line
1 matches a '1'
[0-9]\{1,\} matches one or more digits
\{1,\} matches one or more spaces
K matches a 'K'
>
> Then, in the snd grep command he doubled
> '[0-9]', wouldn't only '^1[0-9]*....'be
> sufficent? Again, 'K' is prefaced with the
> asterisk which doesn't seem necessary to me.
Again, the asterisk applies to the preceding, not following, element.
The example can be written more simply with egrep:
egrep '^1[0-9]+ +K' phonelist.txt
+ means one or more of the preceding element. It's extended regular expression syntax. See "man 7 regex" for basic vs. extended regular expressions.
I hope this helps.
--
Cheers,
Clive
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