Return only matches from substitution in Perl 5.8.8 (was: Perl “p” regex modifier equivalent)

Question

I've got a script (source) to parse svn info to create a suitable string for Bash's $PS1. Unfortunately this doesn't work on one system I'm using which is running Perl 5.8.8 - It outputs all lines instead of only the matches. What would be the Perl 5.8.8 equivalent to the following?

__svn_ps1()
{
    local result=$(
        svn info 2>/dev/null | \
        perl -pe 's;^URL: .*?/((trunk)|(branches|tags)/([^/]*)).*;\2\4 ;p')
    if [ -n "$result" ]
    then
        printf "${1:- (%s)}" $result
    fi  
}

The output from Perl 5.10 contains only a space, parenthesis, one of branch name, tag name or trunk, and the end parenthesis. The output from Perl 5.8.8 (without the final p) contains this plus a parenthesized version of each space-separated part of the svn info output.

A possible workaround involves a simple grep '^URL: ' between the svn and perl commands, but I was hoping to avoid that since this will be executed for each Bash prompt.

Answer 1

If you only want output from a line that matches, don't use the -p command-line switch. It prints the value of $_ at the end of each loop. You might want something with the -n command-line switch:

 perl -ne 'print if s/.../.../'

I'd do it in the same way for Perl v5.8 and v5.10. I'm not sure what you think the /p modifier is doing since you don't use the $`, $&, or $' variables or their per-match equivalents.

You can read about the command-line switches in perlrun.

Answer 2

As of perl 5.10, the /p switch tells perl to put matched content into ${^PREMATCH}, ${^MATCH} and ${^POSTMATCH}.

And the one-liner you've posted never uses those vars, so omit the /p.

UPDATE: Trying to keep up with the initial question...

perl -ne 's/search/replace/ and print'

Will only print lines for which the replacement was made. Note -n versus -p switch. Also, I've tried the -p /p combo on my 5.10 and it happily prints unaltered non-matching lines too. Maybe I missed something...