What Is a Regular Expression?

A regular expression (also called a regex or regexp) is a pattern that can match a piece of text.
The simplest form of regular expression is just a plain string, which matches itself.



A regexp can match more than one string, and you create such a pattern by using some special
characters. For example, the period character (dot) matches any character (except a newline),
so the regular expression '.ython' would match both the string 'python' and the string
'jython'. It would also match strings such as 'qython', '+ython', or ' ython' (in which the first
letter is a single space), but not strings such as 'cpython' or 'ython' because the period matches
a single letter, and neither two nor zero.

    and this period is called a wildcard.

(pattern) ? pattern is used to Escaping Special Characters
 (pattern)* pattern is repeated zero or more times
(pattern)+ pattern is repeated one or more times
(pattern){m,n} pattern is repeated from m to n times
(pattern) ('^') 

pattern is  

 to mark the beginning 

(pattern) 

('$') 

pattern is  

 to mark the 

end of a string

So, for example,

'(http://)?(www\.)?

google

.com'  

would match all of the following strings (and nothing else):

'http://www.google.com'  '

http://google.com'   

'www.google.com' 

'google.com'

 r'w*\

google

.org' matches 'www.google.com', but also '.google.com',
'ww.google.com', and 'wwwwwww.google.com'.

Similarly, r'w+\.python\.org' matches 

'w.google.com' but not '.google.com', 

and r'w{3,4}\.python\.org' matches only 

'www.google.com' and 'wwww.google.com'

'^ht+p' would match'http://

google.com' (and 'htttttp://

google.com

', for that matter) but not 'www.http.org'.

example:

pattern= r'[.?\-",]+'

pattern=r '[a-zA-Z]+'

pattern=r'\*([^\*]+)\*'

pattern=r'[a-z\-\.]+@[a-z\-\.]+'