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    $var !~ /foo/;

C<m/pattern/msixpogcdualn> searches a string for a pattern match,
applying the given options.

    m  Multiline mode - ^ and $ match internal lines
    s  match as a Single line - . matches \n
    i  case-Insensitive
    x  eXtended legibility - free whitespace and comments
    p  Preserve a copy of the matched string -
       ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
    o  compile pattern Once
    g  Global - all occurrences
    c  don't reset pos on failed matches when using /g
    a  restrict \d, \s, \w and [:posix:] to match ASCII only
    aa (two a's) also /i matches exclude ASCII/non-ASCII
    l  match according to current locale
    u  match according to Unicode rules
    d  match according to native rules unless something indicates
       Unicode
    n  Non-capture mode. Don't let () fill in $1, $2, etc...

If 'pattern' is an empty string, the last I<successfully> matched
regex is used. Delimiters other than '/' may be used for both this
operator and the following ones. The leading C<m> can be omitted
if the delimiter is '/'.

C<qr/pattern/msixpodualn> lets you store a regex in a variable,
or pass one around. Modifiers as for C<m//>, and are stored
within the regex.

C<s/pattern/replacement/msixpogcedual> substitutes matches of
'pattern' with 'replacement'. Modifiers as for C<m//>,
with two additions:

    e  Evaluate 'replacement' as an expression
    r  Return substitution and leave the original string untouched.

'e' may be specified multiple times. 'replacement' is interpreted
as a double quoted string unless a single-quote (C<'>) is the delimiter.

C<m?pattern?> is like C<m/pattern/> but matches only once. No alternate
delimiters can be used.  Must be reset with reset().

=head2 SYNTAX

 \       Escapes the character immediately following it
 .       Matches any single character except a newline (unless /s is
           used)
 ^       Matches at the beginning of the string (or line, if /m is used)
 $       Matches at the end of the string (or line, if /m is used)
 *       Matches the preceding element 0 or more times
 +       Matches the preceding element 1 or more times
 ?       Matches the preceding element 0 or 1 times
 {...}   Specifies a range of occurrences for the element preceding it
 [...]   Matches any one of the characters contained within the brackets
 (...)   Groups subexpressions for capturing to $1, $2...
 (?:...) Groups subexpressions without capturing (cluster)
 |       Matches either the subexpression preceding or following it
 \g1 or \g{1}, \g2 ...    Matches the text from the Nth group
 \1, \2, \3 ...           Matches the text from the Nth group
 \g-1 or \g{-1}, \g-2 ... Matches the text from the Nth previous group
 \g{name}     Named backreference
 \k<name>     Named backreference
 \k'name'     Named backreference
 (?P=name)    Named backreference (python syntax)

=head2 ESCAPE SEQUENCES

These work as in normal strings.

   \a       Alarm (beep)
   \e       Escape
   \f       Formfeed
   \n       Newline
   \r       Carriage return
   \t       Tab
   \037     Char whose ordinal is the 3 octal digits, max \777
   \o{2307} Char whose ordinal is the octal number, unrestricted
   \x7f     Char whose ordinal is the 2 hex digits, max \xFF
   \x{263a} Char whose ordinal is the hex number, unrestricted
   \cx      Control-x
   \N{name} A named Unicode character or character sequence
   \N{U+263D} A Unicode character by hex ordinal

   \l  Lowercase next character
   \u  Titlecase next character
   \L  Lowercase until \E
   \U  Uppercase until \E
   \F  Foldcase until \E
   \Q  Disable pattern metacharacters until \E
   \E  End modification

For Titlecase, see L</Titlecase>.

This one works differently from normal strings:

   \b  An assertion, not backspace, except in a character class

=head2 CHARACTER CLASSES

   [amy]    Match 'a', 'm' or 'y'
   [f-j]    Dash specifies "range"
   [f-j-]   Dash escaped or at start or end means 'dash'
   [^f-j]   Caret indicates "match any character _except_ these"

The following sequences (except C<\N>) work within or without a character class.
The first six are locale aware, all are Unicode aware. See L<perllocale>
and L<perlunicode> for details.

   \d      A digit
   \D      A nondigit
   \w      A word character
   \W      A non-word character
   \s      A whitespace character
   \S      A non-whitespace character
   \h      A horizontal whitespace
   \H      A non horizontal whitespace
   \N      A non newline (when not followed by '{NAME}';;
           not valid in a character class; equivalent to [^\n]; it's
           like '.' without /s modifier)
   \v      A vertical whitespace
   \V      A non vertical whitespace