Regular Expressions 101

Explanation
Pro

^start of line

Positive Lookahead(?=(?:.*(?<!\d)1(?!\d)){3})

Non-Capturing Group(?:.*(?<!\d)1(?!\d)){3}

{3}repeats the group contents below exactly 3 times:

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

Negative Lookbehind(?<!\d)

\da Unicode decimal digit

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

Negative Lookahead(?!\d)

\da Unicode decimal digit

Positive Lookahead(?=.*(?<!\d)2(?!\d))

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

Negative Lookbehind(?<!\d)

\da Unicode decimal digit

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Negative Lookahead(?!\d)

\da Unicode decimal digit

.+any Unicode 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

^(?=(?:.*(?<!\d)1(?!\d)){3})(?=.*(?<!\d)2(?!\d)).+$

New

a 1 b 1 c 1 d 2
asdf 2 asdf 1 asdf 1 asdf 1
a 1 b 1 c 2 1
ab1c:1, cb2a:2, ca2c:1, 1