Simple search and replace without regex - perl

Question

I've got a file with various wildcards in it that I want to be able to substitute from a (Bash) shell script. I've got the following which works great until one of the variables contains characters that are special to regexes:
VERSION="1.0"
perl -i -pe "s/VERSION/${VERSION}/g" txtfile.txt # No problems here
APP_NAME="../../path/to/myapp"
perl -i -pe "s/APP_NAME/${APP_NAME}/g" txtfile.txt # Error!
So instead I want something that just performs a literal text replacement rather than a regex. Are there any simple one-line invocations with Perl or another tool that will do this?

Answer 1

The 'proper' way to do this is to escape the contents of the shell variables so that they aren't seen as special regex characters. You can do this in Perl with \Q, as in
s/APP_NAME/\Q${APP_NAME}/g
but when called from a shell script the backslash must be doubled to avoid it being lost, like so
perl -i -pe "s/APP_NAME/\\Q${APP_NAME}/g" txtfile.txt
But I suggest that it would be far easier to write the entire script in Perl

Answer 2

Use the following:
perl -i -pe "s|APP_NAME|\\Q${APP_NAME}|g" txtfile.txt
Since a vertical bar is not a legal character as part of a path, you are good to go.

Answer 3

I don't particularly like this answer because there should be a better way to do a literal replace in Perl. \Q is cryptic. Using quotemeta adds extra lines of code.
But... You can use substr to replace a portion of a string.
#!/usr/bin/perl
my $name = "Jess.*";
my $sentence = "Hi, my name is Jess.*, dude.\n";
my $new_name = "Prince//";
my $name_idx = index $sentence, $name;
if ($name_idx >= 0) {
substr($sentence, $name_idx, length($name), $new_name);
}
print $sentence;
Output:
Hi, my name is Prince//, dude.

Answer 4

You don't have to use a regular expression for this (using substr(), index(), and length()):
perl -pe '
foreach $var ("VERSION", "APP_NAME") {
while (($i = index($_, $var)) != -1) {
substr($_, $i, length($var)) = $ENV{$var};
}
}
'
Make sure you export your variables.

Answer 5

You can use a regex but escape any special characters.
Something like this may work.
APP_NAME="../../path/to/myapp"
APP_NAME=`echo "$APP_NAME" | sed -e '{s:/:\/:}'`
perl -i -pe "s/APP_NAME/${APP_NAME}/g" txtfile.txt

Answer 6

Use:
perl -i -pe "\$r = qq/\Q${APP_NAME}\E/; s/APP_NAME/\$r/go"
Rationale: Escape sequences

Answer 7

I managed to get a working solution, partly based on bits and pieces from other peoples' answers:
app_name='../../path/to/myapp'
perl -pe "\$r = q/${app_name//\//\\/}/; s/APP_NAME/\$r/g" <<<'APP_NAME'
This creates a Perl variable, $r, from the result of the shell parameter expansion:
${app_name//\//\\/}
${ # Open parameter expansion
app_name # Variable name
// # Start global substitution
\/ # Match / (backslash-escaped to avoid being interpreted as delimiter)
/ # Delimiter
\\/ # Replace with \/ (literal backslash needs to be escaped)
} # Close parameter expansion
All that work is needed to prevent forward slashes inside the variable from being treated as Perl syntax, which would otherwise close the q// quotes around the string.
In the replacement part, use the variable $r (the $ is escaped, to prevent it from being treated as a shell variable within double quotes).
Testing it out:
$ app_name='../../path/to/myapp'
$ perl -pe "\$r = q/${app_name//\//\\/}/; s/APP_NAME/\$r/g" <<<'APP_NAME'
../../path/to/myapp