I am having a terrible time with learning Perl regular expressions. I am trying to :
# at the beginning of a line with: #####.# characters (ignoring leading or trailing spaces) with# ---------- #.I know its s/# but that's all I know and all I can find. Any suggestions.
The beginning of a line is matched by ^. Therefore, a line starting with a # is matched by
/^#/
If you want the # to be single, i.e. not followed by another #, you must add a negative character class:
/^#[^#]/
We do not want to replace the character following the #, so we will replace it with a non matching group (called negative look-ahead):
/^#(?!#)/
To add the replacement, just change it to
s/^#(?!#)/#####/
The full line can be matched by the following regular expression:
/^#+$/
Plus means "once or more", ^ and $ have already been explained. We just have to ignore the leading and trailing spaces (* means "zero or more"):
/^ *#+ *$/
We do not want the spaces to be replaced, so we have to keep them. Parentheses create "capture groups" that are numbered from 1:
s/^( *)#+( *)$/$1# ---------- #$2/
s/^#([^#])|^#$/#####\1/
/^#(?!#)/ is more correct for the first one than /^#(?=[^#])/For your first replacement:
$line =~ s/^#/#####/;
The idea here is that you want any line that starts with a '#'. The '^' in the regex says that what follows must be at the beginning of the string.
And for your second replacement:
$line =~ s/^#+$/# ---------- #/;
This uses '^' again and '$'. The '$' at the end says that what comes before must go to the end of the string. '#+' says that there must be one or more '#' characters. So, in other words, the entire string must consist of '#'.
Here's a test script and run:
$ cat foo.pl
#! /usr/bin/perl
use strict;
use warnings;
my @lines = (
"foo line",
"# single comment",
"another line",
"#############",
"# line",
"############",
);
foreach my $line( @lines ){
print "ORIGINAL: $line\n";
$line =~ s/^#/#####/;
$line =~ s/^#+$/# ---------- #/;
print "NEW: $line\n";
print "\n";
}
$ ./foo.pl
ORIGINAL: foo line
NEW: foo line
ORIGINAL: # single comment
NEW: ##### single comment
ORIGINAL: another line
NEW: another line
ORIGINAL: #############
NEW: # ---------- #
ORIGINAL: # line
NEW: ##### line
ORIGINAL: ############
NEW: # ---------- #
## Double comment shouldn't be replaced if I understand the question correctly. If this is supposed to be a "comment beautifier" then the person writing the question might not have considered the possibility of line starting with two hashes.