trim the blank lines...regex

dani_cv has asked for the wisdom of the Perl Monks concerning the following question:

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Re: trim the blank lines...regex by kyle (Abbot) on Apr 11, 2008 at 18:58 UTC
`my $str = "bla bla bla : ==bla bla ==bla bla ====bla bla bla .... .... "; $str =~ s/\n\n+/\n/g; print qq{\$str = "$str";\n}; __END__ $str = "bla bla bla : ==bla bla ==bla bla ====bla bla bla .... .... ";` [download]	[reply] [d/l]
Re^2: trim the blank lines...regex by dani_cv (Acolyte) on Apr 11, 2008 at 21:21 UTC
Thanks a lot...works perfect!. Dan.	[reply]
Re: trim the blank lines...regex by FunkyMonk (Bishop) on Apr 11, 2008 at 18:54 UTC
Have a look at `s///` in Regexp Quote Like Operators in perlop	[reply] [d/l] [select]
Re: trim the blank lines...regex by pc88mxer (Vicar) on Apr 11, 2008 at 19:06 UTC
`$str =~ s/^\s*\n//mg;` or `$str =~ s/^\n//mg` depending on your definition of a blank line. Perhaps a hangup here is that `s/^$//` will match a blank line but will not remove the ending newline since `$` doesn't include the newline.	[reply] [d/l] [select]
Re: trim the blank lines...regex by mr_mischief (Monsignor) on Apr 11, 2008 at 23:23 UTC
You've already been given good advice, including RTFM which really is good advice here. Yet in the spirit of TIMDOWTDI (some of which don't involve regexes)... `while ( <> ) { chomp; print "$_\n" if length $_; }` [download] `{ local $/; my $contents = <>; $contents =~ tr/\n/\n/s; print $contents; }` [download] If you're playing golf: `$/='';chomp,print"$_\n"while<>` [download] `print/./?$_:''while<>` [download] `print/./?$_:''for<>` [download] If you're using one-liners: `perl -ne "print if/./"` [download] `perl -0pe "y/\n/\n/s"` [download] `perl -ne "/./&&print"` [download]	[reply] [d/l] [select]
Re: trim the blank lines...regex by locked_user sundialsvc4 (Abbot) on Apr 11, 2008 at 21:24 UTC
In an arch-typical file-processing loop (such as the ones described in, ummm, the multitudinous `perlfaq`s...), the usual procedure is to use `chomp` to remove any newlines, then use a regular-expression to eliminate unwanted lines. Then output the ones that pass, remembering to include the record-separator. For such a trivial task as this, the `grep` command-line tool might already be exactly what you need. Otherwise, your regular expression is probably going to be something like: `/^\s$/` which breaks-down as follows: The "`^`" character marks the beginning of the line, and the "`$`" marks the end. These are used to bind the enclosing terms to “the entire record,” so that you are searching for records which consist exclusively* of what's between. The `\s` is a character-class reference: anything that qualifies as “white space.” Finally, the "``" means “zero-or-more of the foregoing.” So this regular-expression will match a record that consists entirely of zero-or-more whitespace. As I previously mentioned, you can probably do the entire job with the `grep` command, using the "`-v`" modifier to invert* the meaning of the pattern: all lines which do not match the pattern will be output. This omits the blank lines (which are the ones that match) and outputs the non-blanks. Q.E.D. (An equivalent Perl program could of-course be constructed, but since this is trivial ...)
Re: trim the blank lines...regex by apl (Monsignor) on Apr 11, 2008 at 19:00 UTC
You're asking about the core of Perl, regular expressions. What have you tried so far? Have you asked your teacher for additional help?	[reply]
Re: trim the blank lines...regex by Anonymous Monk on Apr 11, 2008 at 20:10 UTC
Have you independently confirmed that these really are multiple newlines? Perhaps each line starts with "==bla bla" but is then followed by hundreds of blanks, so your editor is simply line wrapping until it finds a newline char? Some editors (e.g. vi) would display such lines very much as you describe. It's always worthwhile to start with the obvious.	[reply]
Re: trim the blank lines...regex by ww (Archbishop) on Apr 11, 2008 at 19:00 UTC
and, please, have a look at Writeup Formatting Tips re using <code> or <c> tags!	[reply]
Re: trim the blank lines...regex by wade (Pilgrim) on Apr 11, 2008 at 19:37 UTC
I'm a little afraid of answering this 'cause it looks a bit like homework. That said... The first question I have is "how did you get the string in the first place?" The answer to this might present a better solution (e.g., don't concatenate blank lines to `$str` in the firs place). To answer your original question, though: `use strict; use warnings; use diagnostics; print "START: '$str'"; $str =~ s/\n\n+/\n/g; print "\nEND: '$str'";` [download] -- Wade	[reply] [d/l] [select]