How to use negative lookahead (NOT lookbehind) to match a string that does NOT contain a given substring at a specific position?

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I'd like to match certain file types (e.g., ".txt") with a non-empty root name that doesn't end in a particular substring (e.g., "-bad"). With negative lookbehind support, the solution is simple:

/.(?<!-bad)\.txt$/

Safari, however, still does not support negative lookbehinds. Ugh. How could I achieve the same result using a negative lookahead? I'd like to do this with a single regular expression. Please do not provide any non-regex or multi-step solutions.

The test code below shows that I'm close, but one test is still failing. 

const regex = /.((?!-bad).{4})\.txt$/;
const tests = [
  ['this-file-bad.txt', false],
  ['this-file.txt', true],
  ['.txt', false],
  ['f.txt', true]
];

const results = tests.map(([input, expected]) => ((regex.test(input) === expected) ? '✅' : '❌') + input);
console.log(results.join('\n'));

regex-lookarounds regex-negation safari javascript regex
1 Answers

You can use

^(?!.*\-bad\.txt$).+\.txt$

Demo1

The regular expression reads as follows:

  • Match the beginning of the string (^)
  • Use a negative lookahead ((?!...)) to assert that the string does not end "-bad.txt"
  • Match one or more characters followed by ".txt" at the end of the string.

To check for any of several file suffices at the same time (which, based on your comment below, may be helpful) you could write, for example:

^(?=.*\.(txt|pdf|csv|docx|html|jpeg)$)(?!.*\-bad\.\1$).+\.\1$

The positive lookahead at the beginning has the sole purpose of capturing the file suffix at the end of the string so that a back-reference can be used in the negative lookahead and at the end.

Demo2

user_2569700
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