Regular Expression

|                          : or (e.g. ab|cd will match in either case of ab or cd)
m/ / 
                    : container of the regular expression
*                          : greedy quantifier
*?                        : non-greedy quantifier
$_ = "I thought you said Fred and <BOLD>Velma</BOLD>, not <BOLD>Wilma</BOLD>";
s#<BOLD>(.*?)</BOLD>#$1#g;    # would match from the first <BOLD> to the last </BOLD>, leaving intact the ones in the middle of the line, hence:
+                         : greedy quantifier
+?                       : non-greedy quantifier
??                       : non-greedy quantifier
.                          : every character, except new line. to include it e.g. [\d\D] :every digit or every non digit: {{.} U{new-line}} OR m//s
()                         : grouping
(){,}                      : /(fred){3,}/    :count. e.g. fredfredfred
(){5,10}?               : non-greedy quantifier
()  \1 (OR $1)        : \1 content of first parenthesis   e.g.   /(.)\1/   means two similar characters --group number is the order of the opening parenthesis
()   \g{N}               :same as above
$_ = "Hello there, neighbor";
if (/(\S+) (\S+), (\S+)/) {
  print "words were $1 $2 $3\n";
These match variables generally stay around until the next successful pattern match. This correctly implies that you shouldn’t use these match variables unless the match succeeded; otherwise, you could be seeing a memory from some previous pattern.
if ($wilma =~ /(\w+)/) {
  print "Wilma's word was $1.\n";
} else {
  print "Wilma doesn't have a word.\n";
(?:   )                     : non-capturing parenthesis (will not be counted for when refering to parentheses. adding or changing these type of parenthesis will not change the parentheses numbers we have already refered to.)
if (/(?:bronto)?saurus (?:BBQ )?(steak|burger)/) {#you can add or remove any of non-capturing parentheses without worrying bout numbers used before
    print "Fred wants a $1\n";
(?<LABEL>PATTERN)       $+{LABEL}    \g{label}(used for back reference)  named captures
my $names = 'Fred or Barney';
if( $names =~ m/(?<name1>\w+) (?:and|or) (?<name2>\w+)\g{name1}/ ) {
    say "I saw $+{name1} and $+{name2}";

/(fred)*/           matches strings like hello, world, even empty string
|                             : or
[ ]                           : character class  e.g. [a-zA-Z]
[^dcf]                      : all chracters except those three    [^n\-z]   matches any character except for n, hyphen, or z
(Note that the hyphen is backslashed because it’s special inside a character class. But the first hyphen in /HAL-[0-9]+/ doesn’t need
^                            : caret anchor: marks the beginning of the string
As the first character of a character class, it negates the class. But outside of a character class, it’s a metacharacter in a different way, being the start-of-string anchor. There are only so many characters, so you have to use some of them twice.
$                            : marks the end of string
          /^fred$/ will match either "fred" or "fred\n" with equal ease.

Character class shortcuts
 \d [0-9]
 \w [a-zA-Z_0-9]
 \s [ \n\t\f\r\] space,new line, tab, form feed, carriage return
 \D [^\d]
 \W [^\w]
 \S [^\s]
 \b word boundary

qw//                       : trim
//                         : Pattern Match 
m, fred, , m/fred/, m{fred}, m<fred>, m[fred] , m^fred^, m!fred!
like the usage of qw    --    The shortcut is that if you choose the forward slash as the delimiter, you may omit the initial m. So, just /fred/ would work
m//i                        : ignore case flag
m//s                       : dot (.) matches also newline flag
m//x                       : added whitesace flag
m//m                      : ^ and $ instead of referring to the begin and end of whole string will refer to begin and end of any new line in the string
open FILE, $filename      #this example appends the name of the file to the beginning of each line.
  or die "Can't open '$filename': $!";
my $lines = join '', <FILE>;     # combine all lines in one variable
$lines =~ s/^/$filename: /gm
($first, $second, $third) = m/(\S+ \S+ \S+) /;  : m in list context
When you use split, the pattern specifies the separator: the part that isn’t the useful data. Sometimes it’s easier to specify what you want to keep.
my $text = "Fred dropped a 5 ton granite block on Mr. Slate";
my @words = ($text =~ /([a-z]+)/ig);
print "Result: @words\n";
# Result: Fred dropped a ton granite block on Mr Slate
string =~ regularExp : Matching against $_ is merely the default; the binding operator, =~, tells Perl to match the pattern on the right against the string on the left, instead of matching against $_.
if ("Hello there, neighbor" =~ /\s(\w+),/) {
       print "That actually matched '$&'.\n";
$&                          : The part of the string that actually matched the pattern is automatically stored in $&
$`                            : whatever came before the matched section
$'                            : whatever was after the matched section
while (<>) {                       # take one input line at a time
    print "Matched: |$`<$&>$'|\n"; # the special match vars
  } else {
    print "No match: |$_|\n";

The regular expression is double-quote interpolated, just as if it were a double-quoted string.

s///g                        : replace (all) (s/// replace only one occurance) (g is the all occurrence flag)
s//\U/  AND   s//\L/   : substitute change case     \u & \l lower case will only affect next character.
@fields = split /separator/, $string;
$result = join $glue, @pieces;    
          # $x = join ":", 4, 6, 8, 10, 12; # $x is "4:6:8:10:12"
my @values = split /:/, $x; # @values is (4, 6, 8, 10, 12)
my $z = join "-", @values;  # $z is "4-6-8-10-12"

$str =~
s/<\s*a\s*href\s*=\s*"mailto:((.*?@.*?\..*?))"[^>]*?>[^<]*?<\/s*a\s*>/\{_email href="$1"_\} $1\{_email\/_}/sig;
if I didn't escape the { it would give me a bare word problem. it recognized it as a variable

Not String

In case you don't want to have a string in your strings, try to draw the Finite State Machine of that string along with a Dead State for the case that don't match that string, then once yo are done make finite states as dead states and make the previous dead state as a finite state.

E.g. to remove <script> tag and any thing that goes in there which could also include other tags use:
$row = preg_replace('/<script>([^<]|<[^\/]|<\/[^s]|<\/s[^c]|<\/sc[^r]|<\/scr[^i]|<\/scri[^p]|<\/scrip[^t]|<\/script[^>])*<\/script>/', '', $row);

Subpages (1): Examples