Char    Meaning

^     beginning of string
$     end of string
.     any character except newline
*     match 0 or more times
+     match 1 or more times
?     match 0 or 1 times
|     alternative (OR)
( )     grouping
[ ]     set of characters
{ }     repetition modifier
\     quote or special


a*    zero or more a’s
a+    one or more a’s
a?    zero or one a’s (i.e., optional a)
a{m}    exactly m a’s
a{m,}    at least m a’s
a{m,n}    at least m but at most n a’s


\t     tab
\n     newline
\r     return (CR)
\xhh     character with hex. code hh
    

\b     "word" boundary
\B     not a "word" boundary


\w     matches any single character classified as a "word" character (alphanumeric or "_")
\W     matches any non-"word" character
\s     matches any whitespace character (space, tab, newline)
\S     matches any non-whitespace character
\d     matches any digit character, equiv. to [0-9]
\D     matches any non-digit character


*** Examples ***
Expression     Matches...
abc         abc (that exact character sequence, but anywhere in the string)
^abc         abc at the beginning of the string
abc$         abc at the end of the string
a|b         either of a and b
^abc|abc$     the string abc at the beginning or at the end of the string
ab{2,4}c     an a followed by two, three or four b’s followed by a c
ab{2,}c     an a followed by at least two b’s followed by a c
ab*c         an a followed by any number (zero or more) of b’s followed by a c
ab+c         an a followed by one or more b’s followed by a c
ab?c         an a followed by an optional b followed by a c; that is, either abc or ac
a.c         an a followed by any single character (not newline) followed by a c
a\.c         a.c exactly
[abc]         any one of a, b and c
[Aa]bc         either of Abc and abc
[abc]+         any (nonempty) string of a’s, b’s and c’s (such as a, abba, acbabcacaa)
[^abc]+     any (nonempty) string which does not contain any of a, b and c (such as defg)
\d\d         any two decimal digits, such as 42; same as \d{2}
\w+         a "word": a nonempty sequence of alphanumeric characters and low lines (underscores),
            such as foo and 12bar8 and foo_1
100\s*mk      the strings 100 and mk optionally separated by any amount of white space
              (spaces, tabs, newlines)
abc\b         abc when followed by a word boundary (e.g. in abc! but not in abcd)
perl\B         perl when not followed by a word boundary (e.g. in perlert but not in perl stuff)