正则表达式

. - Any Character Except New Line
\d - Digit (0-9)
\D - Not a Digit(0-9)
\w - Word Character (a-z, A-Z, 0-9, _)
\W - Not a word Character
\s - Whitespace (space, tab, newline)
\S - Not Whitespace

\b - Word Boundary
\B - Not a word boundary
^ - Beginning of a String
$ - End of a String

[] - Matches Characters in bracket
[^ ] - Matches Characters NOT in bracket
| - Either Or
() - Group

Quantifiers:
* - 0 or More
+ - 1 or More
? - 0 or 1
{3} - exact number
{3,4} - range of numbers

eg.
email address search expression:

[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+

URL search expression:

https?://(www\.)?(\w+)(\.\w+)

关于URL匹配，网上找到两个比较全的：
1.这个stackoverflow上的我试了蛮好用的

(https?|ftp|file)://[-A-Za-z0-9+&@#/%?=~_|!:,.;]+[-A-Za-z0-9+&@#/%=~_|]

2. 这个更全，不过一般似乎用不着，参看此文的评论区第一条

(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))

原文