Regular Expression (RegEx) - Easy to learn

Regular Expression (RegEx) is a sequence of characters that define a search pattern. Each character in a regular expression is understood to be a metacharacter (with its special meaning), or a regular character (with its literal meaning). For example, if we define a regular expression r. where ‘r’ is a literal character that matches just ‘r’ and ‘.’ is a meta-character that matches every character except a newline.
In JMeter, Regular Expression plays an important role. Regular Expression is used to identify dynamic values that come in a response. In Microfocus LoadRunner, dynamic values are captured by Correlation whereas, in JMeter, these values are extracted by Regular Expression Extractor.

Regular Expression is a pattern that is used to specify a set of strings required for getting dynamic value. To prepare a regular expression, it is important to learn how we can create it using metacharacter and literal characters.

The most important and frequently used Reg-Ex for JMeter is (.+?)

The below table will help you to understand how reg-ex can be created. The blue-coloured text is the extracted value through the related token.

Tokens	Meaning	Example
.	Matches any character other than newline	Statement: Regular Expression is a pattern Token: . Extracted value: Regular Expression is a pattern
.+	Matches any string excluding new line	Statement: Regular Expression is a pattern Token: .+ Extracted value: Regular Expression is a pattern
.*	Matches any string including a new line	Statement: Regular Expression is a pattern Token: .* Extracted value: Regular Expression is a pattern
X.*Y	Matches as many characters as possible.	Statement: Regular Expression is a pattern Token: E.i Extracted value: Regular Expression is a pattern*
(.+?)	Matches the strings including letters, digits and space (exclude new line)	Statement: Regular Expression is a pattern Token: Reg(.+?)pattern Extracted value: Regular Expression is a pattern
()	Matches everything or enclosed character/string	Statement: Regular Expression is a pattern Token: () Extracted value: Regular Expression is a pattern OR Statement: Regular Expression is a pattern Token: (press) Extracted value: Regular Expression is a pattern
\|	OR operation	Statement: Regular Expression is a pattern Token: (Reg\|Ex) Extracted value: Regular Expression is a pattern
a?	Matches an `a` character or nothing	Statement: Regular Expression is a pattern Token: a? Extracted value: Regular Expression is a pattern
a*	Matches zero or more consecutive `a` characters	Statement: Regular Expression is a pattern Token: a* Extracted value: Regular Expression is a pattern
a+	Matches one or more consecutive `a` characters.	Statement: Regular Expression is a pattern Token: a+ Extracted value: Regular Expression is a pattern
{n}	Matches exactly n consecutive characters	Statement: Regular Expression is a pattern Token: s{2} Extracted value: Regular Expression is a pattern
{n,}	Matches at least n consecutive characters	Statement: Regular Expression is a pattern. s ss sss ssss sssss ssssss. Token: s{3} Extracted value: Regular Expression is a pattern. s ss sss ssss sssss ssssss.
{m,n}	Matches between m and n (inclusive) consecutive characters	Statement: Regular Expression is a pattern. s ss sss ssss sssss ssssss. Token: s{2,4} Extracted value: Regular Expression is a pattern. s ss sss ssss sssss ssssss
\*	Escape Character	Statement: Regular Expression is a pattern Token: \\s Extracted value:* Regular Expression is a pattern
[]	Matches specific characters (case-sensitive)	Statement: Regular Expression is a pattern Token: [a] Extracted value: Regular Expression is a pattern or Statement: Regular Expression is a pattern Token: [aer] Extracted value: Regular Expression is a pattern
[^]	Matches any character except the given character	Statement: Regular Expression is a pattern Token: [^a] Extracted value: Regular Expression is a pattern or Statement: Regular Expression is a pattern Token: [^ae] Extracted value: Regular Expression is a pattern
[a-z]	Matches any characters between a and z	Statement: Regular Expression is a pattern Token: [a-z] Extracted value: Regular Expression is a pattern
[^a-z]	Matches any characters except those in the range a-z, even space.	Statement: Regular Expression is a pattern Token: [^a-z] Extracted value: Regular Expression is a pattern
[A-Z]	Matches any characters between A and Z	Statement: Regular Expression is a pattern Token: [A-Z] Extracted value: Regular Expression is a pattern
[^A-Z]	Matches any characters except those in the range A-Z, even space.	Statement: Regular Expression is a pattern Token: [^A-Z] Extracted value: Regular Expression is a pattern
[a-zA-Z]	Matches any characters between a-z or A-Z	Statement: Regular Expression is a 123 pattern Token: [a-zA-Z] Extracted value: Regular Expression is a 123 pattern
[^a-zA-Z]	Matches any characters between a-z or A-Z	Statement: Regular Expression is a 123 pattern Token: [^a-zA-Z] Extracted value: Regular Expression is a 123 pattern
\	This is used to match the literal value of any metacharacter	Statement: Regular Expression is a pattern Token: \a Extracted value: Regular Expression is a pattern
\s	Matches any space, tab or newline character	Statement: Regular Expression is a pattern Token: \s Extracted value: Regular_Expression_is_a_pattern *It will capture all the spaces which are represented as _
\S	Matches any character other than space, tab or newline.	Statement: Regular Expression is a pattern Token: \S Extracted value: Regular_Expression_is_a_pattern *It will not capture any space. Just for representation purpose, they are marked as _.
\S+	Matches any string other than a space, tab or newline.	Statement: Regular Expression is a pattern Token: \S+ Extracted value: Regular_Expression_is_a_pattern *It will not capture any space. Just for representation purposes, they are marked as _.
\d or [0-9]	Matches any decimal digit	Statement: Regular Expression is a 123 pattern Token: \d+ Extracted value: Regular Expression is a 123 pattern or Statement: Regular Expression is a 123 pattern Token: [0-9] Extracted value: Regular Expression is a 123 pattern
\D	Matches any character other than a decimal digit	Statement: Regular Expression is a 123 pattern Token: \D Extracted value: Regular Expression is a 123 pattern
\D+	Matches any string other than a decimal digit	Statement: Regular Expression is a 123 pattern Token: \D+ Extracted value: Regular Expression is a 123 pattern
\w or [a-zA-z0-9_]	Matches any letter, digit or underscore (exclude white space)	Statement: Regular Expression is a 123_321 pattern Token: \w Extracted value: Regular Expression is a 123_321 pattern *It will not capture any space.
\W or [^a-zA-Z0-9_]	Matches anything other than a letter, digit or underscore	Statement: Regular Expression is a pattern Token: \W Extracted value: Regular Expression is a 123_321 pattern *It will capture all the spaces.
\v	Matches newlines and vertical tabs
\n	Matches a newline character
\r	Matches a carriage return character
\t	Matches a tab character
\0	Matches a null character
[\b]	Matches the backspace control character.
(.)\	Usually referred to as a `backreference`, this will match a repeat of the text captured in a previous set of parentheses	Statement: Regular Expression is a pattern Token: (.)\a Extracted value: Regular Expression is a pattern Matched: ‘la’, ‘ a’ and ‘pa’
\uYYYY	Matches the Unicode character with the given hex value
\xYY	Matches the 8-bit character with the given hex value
\ddd	Matches the 8-bit character with the given octal value.
\cY	Matches ASCII characters

You can build Reg-Ex using online tools available in the market. I have provided some useful Regular Expression Extractors/Testers links below: