Match whitespace but not newlines

I sometimes want to match whitespace but not newline.

So far I've been resorting to [ \t]. Is there a less awkward way?

Use a double-negative:

/[^\S\r\n]/

That is, not-not-whitespace (the capital S complements) or not-carriage-return or not-newline. Distributing the outer not (i.e., the complementing ^ in the character class) with De Morgan's law, this is equivalent to “whitespace but not carriage return or newline.” Including both \r and \n in the pattern correctly handles all of Unix (LF), classic Mac OS (CR), and DOS-ish (CR LF) newline conventions.

No need to take my word for it:

#! /usr/bin/env perl

use strict;
use warnings;

use 5.005;  # for qr//

my $ws_not_crlf = qr/[^\S\r\n]/;

for (' ', '\f', '\t', '\r', '\n') {
  my $qq = qq["$_"];
  printf "%-4s => %s\n", $qq,
    (eval $qq) =~ $ws_not_crlf ? "match" : "no match";
}

Output:

" "  => match
"\f" => match
"\t" => match
"\r" => no match
"\n" => no match

Note the exclusion of vertical tab, but this is addressed in v5.18.

Before objecting too harshly, the Perl documentation uses the same technique. A footnote in the “Whitespace” section of perlrecharclass reads

Prior to Perl v5.18, \s did not match the vertical tab. [^\S\cK] (obscurely) matches what \s traditionally did.

The same section of perlrecharclass also suggests other approaches that won’t offend language teachers’ opposition to double-negatives.

Outside locale and Unicode rules or when the /a switch is in effect, “\s matches [\t\n\f\r ] and, starting in Perl v5.18, the vertical tab, \cK.” Discard \r and \n to leave /[\t\f\cK ]/ for matching whitespace but not newline.

If your text is Unicode, use code similar to the sub below to construct a pattern from the table in the aforementioned documentation section.

sub ws_not_nl {
  local($_) = <<'EOTable';
0x0009        CHARACTER TABULATION   h s
0x000a              LINE FEED (LF)    vs
0x000b             LINE TABULATION    vs  [1]
0x000c              FORM FEED (FF)    vs
0x000d        CARRIAGE RETURN (CR)    vs
0x0020                       SPACE   h s
0x0085             NEXT LINE (NEL)    vs  [2]
0x00a0              NO-BREAK SPACE   h s  [2]
0x1680            OGHAM SPACE MARK   h s
0x2000                     EN QUAD   h s
0x2001                     EM QUAD   h s
0x2002                    EN SPACE   h s
0x2003                    EM SPACE   h s
0x2004          THREE-PER-EM SPACE   h s
0x2005           FOUR-PER-EM SPACE   h s
0x2006            SIX-PER-EM SPACE   h s
0x2007                FIGURE SPACE   h s
0x2008           PUNCTUATION SPACE   h s
0x2009                  THIN SPACE   h s
0x200a                  HAIR SPACE   h s
0x2028              LINE SEPARATOR    vs
0x2029         PARAGRAPH SEPARATOR    vs
0x202f       NARROW NO-BREAK SPACE   h s
0x205f   MEDIUM MATHEMATICAL SPACE   h s
0x3000           IDEOGRAPHIC SPACE   h s
EOTable

  my $class;
  while (/^0x([0-9a-f]{4})\s+([A-Z\s]+)/mg) {
    my($hex,$name) = ($1,$2);
    next if $name =~ /\b(?:CR|NL|NEL|SEPARATOR)\b/;
    $class .= "\\N{U+$hex}";
  }

  qr/[$class]/u;
}

Other Applications

The double-negative trick is also handy for matching alphabetic characters too. Remember that \w matches “word characters,” alphabetic characters and digits and underscore. We ugly-Americans sometimes want to write it as, say,

if (/[A-Za-z]+/) { ... }

but a double-negative character-class can respect the locale:

if (/[^\W\d_]+/) { ... }

Expressing “a word character but not digit or underscore” this way is a bit opaque. A POSIX character-class communicates the intent more directly

if (/[[:alpha:]]+/) { ... }

or with a Unicode property as szbalint suggested

if (/\p{Letter}+/) { ... }

Perl versions 5.10 and later support subsidiary vertical and horizontal character classes, \v and \h, as well as the generic whitespace character class \s

The cleanest solution is to use the horizontal whitespace character class \h. This will match tab and space from the ASCII set, non-breaking space from extended ASCII, or any of these Unicode characters

U+0009 CHARACTER TABULATION
U+0020 SPACE
U+00A0 NO-BREAK SPACE (not matched by \s)

U+1680 OGHAM SPACE MARK
U+2000 EN QUAD
U+2001 EM QUAD
U+2002 EN SPACE
U+2003 EM SPACE
U+2004 THREE-PER-EM SPACE
U+2005 FOUR-PER-EM SPACE
U+2006 SIX-PER-EM SPACE
U+2007 FIGURE SPACE
U+2008 PUNCTUATION SPACE
U+2009 THIN SPACE
U+200A HAIR SPACE
U+202F NARROW NO-BREAK SPACE
U+205F MEDIUM MATHEMATICAL SPACE
U+3000 IDEOGRAPHIC SPACE

The vertical space pattern \v is less useful, but matches these characters

U+000A LINE FEED
U+000B LINE TABULATION
U+000C FORM FEED
U+000D CARRIAGE RETURN
U+0085 NEXT LINE (not matched by \s)

U+2028 LINE SEPARATOR
U+2029 PARAGRAPH SEPARATOR

There are seven vertical whitespace characters which match \v and eighteen horizontal ones which match \h. \s matches twenty-three characters

All whitespace characters are either vertical or horizontal with no overlap, but they are not proper subsets because \h also matches U+00A0 NO-BREAK SPACE, and \v also matches U+0085 NEXT LINE, neither of which are matched by \s

A variation on Greg’s answer that includes carriage returns too:

/[^\S\r\n]/

This regex is safer than /[^\S\n]/ with no \r. My reasoning is that Windows uses \r\n for newlines, and Mac OS 9 used \r. You’re unlikely to find \r without \n nowadays, but if you do find it, it couldn’t mean anything but a newline. Thus, since \r can mean a newline, we should exclude it too.

The below regex would match white spaces but not of a new line character.

(?:(?!\n)\s)

DEMO

If you want to add carriage return also then add \r with the | operator inside the negative lookahead.

(?:(?![\n\r])\s)

DEMO

Add + after the non-capturing group to match one or more white spaces.

(?:(?![\n\r])\s)+

DEMO

I don't know why you people failed to mention the POSIX character class [[:blank:]] which matches any horizontal whitespaces (spaces and tabs). This POSIX chracter class would work on BRE(Basic REgular Expressions), ERE(Extended Regular Expression), PCRE(Perl Compatible Regular Expression).

DEMO