regex101: PCRE2 (PHP) Match Workspace

^start of line

Non-Capturing Group(?:(?!.*\bfoo\b)|(?=(?:(?!\bfoo\b).)*\bfoo\b(?!(.*\bfoo\b))))

1st Alternative(?!.*\bfoo\b)

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

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

\ban ASCII word boundary

foothe literal text foo (case sensitive)

\ban ASCII word boundary

2nd Alternative(?=(?:(?!\bfoo\b).)*\bfoo\b(?!(.*\bfoo\b)))

Positive Lookahead(?=(?:(?!\bfoo\b).)*\bfoo\b(?!(.*\bfoo\b)))

Non-Capturing Group(?:(?!\bfoo\b).)*

*repeats the group contents below any number of times:

Negative Lookahead(?!\bfoo\b)

\ban ASCII word boundary

foothe literal text foo (case sensitive)

\ban ASCII word boundary

.any character (except for line terminators)

\ban ASCII word boundary

foothe literal text foo (case sensitive)

\ban ASCII word boundary

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

Group 1(.*\bfoo\b)

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

\ban ASCII word boundary

foothe literal text foo (case sensitive)

\ban ASCII word boundary

Non-Capturing Group(?:(?!.*\ba\b)|(?=(?:(?!\ba\b).)*\ba\b(?!(.*\ba\b))))

1st Alternative(?!.*\ba\b)

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

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

\ban ASCII word boundary

athe literal a (97₁₀ / 141₈ / 61₁₆) (case sensitive)

\ban ASCII word boundary

2nd Alternative(?=(?:(?!\ba\b).)*\ba\b(?!(.*\ba\b)))

Positive Lookahead(?=(?:(?!\ba\b).)*\ba\b(?!(.*\ba\b)))

Non-Capturing Group(?:(?!\ba\b).)*

*repeats the group contents below any number of times:

Negative Lookahead(?!\ba\b)

\ban ASCII word boundary

athe literal a (97₁₀ / 141₈ / 61₁₆) (case sensitive)

\ban ASCII word boundary

.any character (except for line terminators)

\ban ASCII word boundary

athe literal a (97₁₀ / 141₈ / 61₁₆) (case sensitive)

\ban ASCII word boundary

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

Group 2(.*\ba\b)

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

\ban ASCII word boundary

athe literal a (97₁₀ / 141₈ / 61₁₆) (case sensitive)

\ban ASCII word boundary

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

25 tokens

Regular Expression

^(?:(?!.*\bfoo\b)|(?=(?:(?!\bfoo\b).)*\bfoo\b(?!(.*\bfoo\b))))(?:(?!.*\ba\b)|(?=(?:(?!\ba\b).)*\ba\b(?!(.*\ba\b))))

New

foo
a
foo a
a foo

foo foo
a a
foo a foo
a foo a
foofoo
aa
afooa

Substitution

foo
a
foo a
a foo

foo foo
a a
foo a foo
a foo a
foofoo
aa
afooa