Regular Expressions 101

Explanation
Pro

^start of line

Non-Capturing Group(?:(?=.*\bbig dog\b)(?!.*\bcat\b)|(?!.*\bbig dog\b)(?=.*\bcat\b))

1st Alternative(?=.*\bbig dog\b)(?!.*\bcat\b)

Positive Lookahead(?=.*\bbig dog\b)

.*any character (except for line terminators), repeated any number of times

\ban ASCII word boundary

big dogthe literal text big dog (case sensitive)

\ban ASCII word boundary

Negative Lookahead(?!.*\bcat\b)

.*any character (except for line terminators), repeated any number of times

\ban ASCII word boundary

catthe literal text cat (case sensitive)

\ban ASCII word boundary

2nd Alternative(?!.*\bbig dog\b)(?=.*\bcat\b)

Negative Lookahead(?!.*\bbig dog\b)

.*any character (except for line terminators), repeated any number of times

\ban ASCII word boundary

big dogthe literal text big dog (case sensitive)

\ban ASCII word boundary

Positive Lookahead(?=.*\bcat\b)

.*any character (except for line terminators), repeated any number of times

\ban ASCII word boundary

catthe literal text cat (case sensitive)

\ban ASCII word boundary

.+any character (except for line terminators), repeated at least once

$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.

25 tokens

^(?:(?=.*\bbig dog\b)(?!.*\bcat\b)|(?!.*\bbig dog\b)(?=.*\bcat\b)).+$

New

That big dog and cat went to town.

The dog and cat went to town.

The dog went to town
The cat went to town