Regex

Match everything but

a string starting with a specific pattern (e.g. any - empty, too - string not starting with foo):
- Lookahead-based solution for NFAs:
  - ^(?!foo).*$
  - ^(?!foo)
- Negated character class based solution for regex engines not supporting lookarounds:
  - ^(([^f].{2}|.[^o].|.{2}[^o]).*|.{0,2})$
  - ^([^f].{2}|.[^o].|.{2}[^o])|^.{0,2}$
a string ending with a specific pattern (say, no world. at the end):
- Lookbehind-based solution:
  - (?<!world\.)$
  - ^.*(?<!world\.)$
- Lookahead solution:
  - ^(?!.*world\.$).*
  - ^(?!.*world\.$)
- POSIX workaround:
  - ^(.*([^w].{5}|.[^o].{4}|.{2}[^r].{3}|.{3}[^l].{2}|.{4}[^d].|.{5}[^.])|.{0,5})$
  - ([^w].{5}|.[^o].{4}|.{2}[^r].{3}|.{3}[^l].{2}|.{4}[^d].|.{5}[^.]$|^.{0,5})$
a string containing specific text (say, not match a string having foo):
- Lookaround-based solution:
  - ^(?!.*foo)
  - ^(?!.*foo).*$
- POSIX workaround: Use the online regex generator at www.formauri.es/personal/pgimeno/misc/non-match-regex
a string containing specific character (say, avoid matching a string having a | symbol): ^[^|]*$
a string equal to some string (say, not equal to foo):
- Lookaround-based:
  - ^(?!foo$)
  - ^(?!foo$).*$
- POSIX: ^(.{0,2}|.{4,}|[^f]..|.[^o].|..[^o])$
a sequence of characters:
- PCRE (match any text but cat): /cat(*SKIP)(*FAIL)|[^c]*(?:c(?!at)[^c]*)*/i or /cat(*SKIP)(*FAIL)|(?:(?!cat).)+/is
- Other engines allowing lookarounds: (cat)|[^c]*(?:c(?!at)[^c]*)* (or (?s)(cat)|(?:(?!cat).)*, or (cat)|[^c]+(?:c(?!at)[^c]*)*|(?:c(?!at)[^c]*)+[^c]*) and then check with language means: if Group 1 matched, it is not what we need, else, grab the match value if not empty
a certain single character or a set of characters: Use a negated character class: [^a-z]+ (any char other than a lowercase ASCII letter) Matching any char(s) but |: [^|]+

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