Regular Expressions 101

(?msx)applies the following effective flags to the remainder of the pattern: gmsx

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

s modifier: single line. Allows a dot to match line terminators

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

Group 1(^http:\/\/www\. .+$)

^start of line

http:the literal text http: (case sensitive)

\/the literal / (47₁₀ / 57₈ / 2F₁₆)

wwwthe literal text www (case sensitive)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

.+any character, repeated at least once

$end of line

Positive Lookahead(?= .* \1)

.*any character, repeated any number of times

\1the text saved by group 1

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

Detailed match information will be displayed here automatically.

25 tokens

Regular Expression

(?msx) 
(^http:\/\/www\. .+$) 
(?= .* \1)

New

http://www.link1.com
http://www.link2.com
http://www.link3.com
http://www.link4.com
http://www.link1.com
http://www.link2.com
http://www.link4.com
http://www.link4.com