Regular Expressions 101

(?s)applies the following effective flags to the remainder of the pattern: gms

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

.*?any character, repeated any number of times

Group 1(<div id="start">.*?<div id="end">)

<div id="start">the literal text <div id="start"> (case sensitive)

.*?any character, repeated any number of times

<div id="end">the literal text <div id="end"> (case sensitive)

Non-Capturing Group(?:(?:(?!<div id="start">).)*$)?

?repeats the group contents below at most once:

Non-Capturing Group(?:(?!<div id="start">).)*

*repeats the group contents below any number of times:

Negative Lookahead(?!<div id="start">)

<div id="start">the literal text <div id="start"> (case sensitive)

.any character

$end of line

Global pattern flags

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

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

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

(?s).*?(<div id="start">.*?<div id="end">)(?:(?:(?!<div id="start">).)*$)?

New

Stuff I dont need before

<div id="start">
blablabla11
blablabla12
<div id="end">

Stuff I dont need in the middle

<div id="start">
blablabla21
blablabla22
<div id="end">

Stuff I dont need in the middle2

<div id="start">
blablabla31
blablabla32
<div id="end">

Stuff I dont need in the end

Substitution

$1\n\n