(?:): Non-Capturing Parentheses. Group but do not capture (Supported by Perl and many other languages).
In Perl regular expressions, \b normally matches a word boundary, but within a character class, it matches a backspace.
Perl can check a string in a variable against a regex using the construct $variable = ̃ m/ regex / . The m indicates that a match is requested, while the slashes delimit (and are not part of) the regular expression. The whole test, as a unit, is either true or false.
Among the more useful shorthands that Perl and many other flavors of regex provide are:
\t: a tab character
\n: a newline character
\r: a carriage-return character
\s: matches any “whitespace” character (space, tab, newline, formfeed, and such)
\S: anything not \s
\w: [a-zA-Z0-9_] (useful as in \w+, ostensibly to match a word)
\W: anything not \w, i.e., [ˆa-zA-Z0-9_]
\d: [0-9] , i.e., a digit
\D: anything not \d, i.e., [ˆ0-9]
Lookaround constructs don’t match text, but rather match positions within the text. They don't “consume” text.
Positive lookahead (one type of lookaround), peeks forward in the text (toward the right) to see if its subexpression can match, and is successful as a regex component if it can. It's specified with the special sequence (?=).
Negative lookahead is exactly the same but it succeeds if it's expression is not able to match. It's sequence is (?!).
Positive lookbehind (another type of lookaround), looks back in the text(toward the left) to see if its subexpression can match, and is successful as a regex component if it can. It's given with the special sequence (?<=).
Negative lookbehind is exactly the same but it succeeds if it's expression is not able to match. It's sequence is (?<!).