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

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