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).*$^(?!foo)
- Lookahead-based solution for NFAs:
- Negated character class based solution for regex engines not supporting lookarounds:
^(([^f].{2}|.[^o].|.{2}[^o]).*|.{0,2})$^([^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})$
- Lookbehind-based solution:
- 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
- Lookaround-based solution:
- 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])$
- Lookaround-based:
- a sequence of characters:
- PCRE (match any text but
cat):/cat(*SKIP)(*FAIL)|[^c]*(?:c(?!at)[^c]*)*/ior/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
- PCRE (match any text but
- 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
|:[^|]+
- Use a negated character class:
Demo note: the newline \n is used inside negated character classes in demos to avoid match overflow to the neighboring line(s). They are not necessary when testing individual strings.
Anchor note: In many languages, use \A to define the unambiguous start of string, and \z (in Python, it is \Z, in JavaScript, $ is OK) to define the very end of the string.
Dot note: In many flavors (but not POSIX, TRE, TCL), . matches any char but a newline char. Make sure you use a corresponding DOTALL modifier (/s in PCRE/Boost/.NET/Python/Java and /m in Ruby) for the . to match any char including a newline.
Backslash note: In languages where you have to declare patterns with C strings allowing escape sequences (like \n for a newline), you need to double the backslashes escaping special characters so that the engine could treat them as literal characters (e.g. in Java, world\. will be declared as "world\\.", or use a character class: "world[.]"). Use raw string literals (Python r'\bworld\b'), C# verbatim string literals @"world\.", or slashy strings/regex literal notations like /world\./.