RegExpTest - Build your regular expression online
an online tool from softlion's webtools collection
A regular expression is a very powerful compact langage for string manipulation. It may be used to
  • verify string formats in user input (email, enforce password policies ...)
  • generically and easily separate and/or replace items in a string (parsing, placeholders ...)
Your regular expression pattern: (line breaks are removed)
↑5  ↑20  |  ↓5  ↓20
or try a pattern template:
IP address v4  test ok  test ko
Email address (full RFC 822)  test ok  test ko

Match options:

Test string:

Regular expressions cheat sheet
• Special characters . * + ? ^ $ { [ ( | ) \ have special meaning and must be escaped with \ to be used without the special meaning : \. \* \+ \? \^ \$ \{ \[ \( \| \) \\

.any char, exactly 1 time
*any char, 0-∞ times
+any char, 1-∞ times
?any char, 0-1 time
^start of string (or line if multiline mode)
$end of string (or line if multiline mode)
|equivalent to OR (there is no AND, check the reverse of what you need for AND)

See here for more details.
• To create a named group (.NET specific), use (?<GroupName>Pattern)
Example to get the 4 digit groups of an IP address: (?<g4>\d{1,3})\.(?<g3>\d{1,3})\.(?<g2>\d{1,3})\.(?<g1>\d{1,3})
• To create a non capturing group (used to optimize resources needed for the computation), use (?:Pattern) instead of (Pattern) which is a capturing group
• Useful link: regular expression implementation in .NET here

Regular expressions tricks
(. may be replaced by any pattern)
.*? finds the smallest interval matching the expression
.* finds the largest interval

.{8,} finds at least 8 times any char
.{8} finds exactly 8 times any char
.{8,12} finds between 8 and 12 times any char

[a-z] finds exactly one time any character between a and z. [a-zA-Z] may be used if ignore case is not used.
[4-6] finds exactly one time any character between 4 and 6.

RFC 822 regular expression is from