Expression Meaning
a Just an 'a' character
. any character except new-line
[bgh.] one of the chars listed in the character class b,g,h or .
[b-h] The same as [bcdefgh]
[a-z] Lower case letters
[b-] The letter b or -
[^bx] Anything except b or x
\w Word characters: [a-zA-Z0-9_]
\d Digits: [0-9]
\s [\f\t\n\r ] form-feed, tab, newline, carriage return and SPACE
\W [^\w]
\D [^\d]
\S [^\s]
[:class:] POSIX character classes (alpha, alnum...)
\p{...} Unicode definitions (IsAlpha, IsLower, IsHebrew, ...)
a* 0-infinite 'a' characters
a+ 1-infinite 'a' characters
a? 0-1 'a' characters
a{n,m} n-m 'a' characters
( ) Grouping and capturing
| Alternation
\1, \2 Capture buffers
$1, $2 Capture variables
^ $ Beginning and end of string ancors
See also perldoc perlre