Regular Expressions 101

^start of line

Non-Capturing Group(?:foo|bar)

1st Alternativefoo

foothe literal text foo (case sensitive)

2nd Alternativebar

barthe literal text bar (case sensitive)

Non-Capturing Group(?: (?:foo|bar))*

*repeats the group contents below any number of times:

the literal (32₁₀ / 40₈ / 20₁₆)

Non-Capturing Group(?:foo|bar)

1st Alternativefoo

foothe literal text foo (case sensitive)

2nd Alternativebar

barthe literal text bar (case sensitive)

$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

^(?:foo|bar)(?: (?:foo|bar))*$

Valid instances are: 
foo
bar
foo foo
foo bar
foo foo bar
foo bar foo bar

Invalid instances: 
foofoo bar
baz foo bar
foo bar baz