Re: [bug-gawk] Lookahead pattern matching

[arnold]

There is no look-ahead pattern matching method, nor is there likely to be.

The way you've done this is pretty standard for awk. IIRC it's referred to
as "control-break programming".

A different option is to read all the lines into an array and then
process them in a loop in the END rule. That may make your code
flow more easily.

As a side point, you would be better off using the -v option to set
the regex variable on the command line than reading it with getline in
the way you've done.

Arnold

Peng Yu <address@hidden> wrote:

> Hi,
>
> I want to look at future lines to determine what to do with the current line.
>
> In the following example, I want to check if the next line starts with
> "a" to decide whether to print the current line with a newline or not.
>
> I think lookahead pattern matching is a general class of problems. But
> the awk code does not look very nice, users need to do the lookahead
> on their own.
>
> Is there a more elegant way to program for such class of problems? Thanks.
>
> $ awk -f ./main.awk <(printf '%s' '^a') <<EOF
> A
> aB
> aC
> D
> EOF
> printf '%s' '^a'
> AaBaC
> D
>
> ########
> BEGIN {
>     OLDRS=RS
>     RS="^$"
>     getline regex < ARGV[1]
>     delete ARGV
>     RS=OLDRS
>     if(getline) {
>         seen_last = 1
>         last = $0
>     }
> }
> {
>     if($0 ~ regex) {
>         printf("%s", last)
>     } else {
>         print last
>     }
>     last = $0
> }
> END {
>     if(seen_last) print last
> }
>
> -- 
> Regards,
> Peng