Aus dem Kochbuch Rezept 6.18 (bzw. 6.17 der ersten Auflage):

True if either /ALPHA/ or /BETA/ matches, like /ALPHA/ || /BETA/:

/ALPHA|BETA/

True if both /ALPHA/ and /BETA/ match, but may overlap, meaning "BETALPHA" should be okay, like /ALPHA/ && /BETA/:

/^(?=.*ALPHA)(?=.*BETA)/s

True if both /ALPHA/ and /BETA/ match, but may not overlap, meaning that "BETALPHA" should fail:

/ALPHA.*BETA|BETA.*ALPHA/s

True if pattern /PAT/ does not match, like $var !~ /PAT/:

/^(?:(?!PAT).)*$/s

True if pattern BAD does not match, but pattern GOOD does:

/(?=^(?:(?!BAD).)*$)GOOD/s\n\n

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