Building a regular expression that tests true for '!=' but false for '!'

Question

I'm looking to allow the inequality operator, but disallow the special character '!' on its own.

Maybe I can use two regular expressions, /!=/ and /!/. I want to test for /!/ if it is alone - and output an error if it isn't accompanied by an equal sign. The issue is that /!/ will test true if the input is '!='. Thoughts?

Input: "!= !"

Expected Output: "Token '!=' Found

Invalid Character '!' Found

Please mention your inputs strings and expected output strings. — Sahil Gulati, Mar 06 '17 at 07:20
Why use a regex for a fixed value? You may just check if there is a `!=`. If you need to check if a string has `!` not followed with `=`, yes, use `/!(?!=)/` regex. — Wiktor Stribiżew, Mar 06 '17 at 07:21
@Azurasky Are you expecting this https://regex101.com/r/74iVXd/1? — Sahil Gulati, Mar 06 '17 at 07:23
I am using a regular expression so that I can test for the presence of these values in a string that could potentially be very long. — Streamer, Mar 06 '17 at 07:23
I still need to check if it contains `!` by itself. Which is an issue because a single exclamation mark is also a part of the inequality operator `!=`. — Streamer, Mar 06 '17 at 07:25
Crusy - can you elaborate on that a little more on that and maybe put it in an answer? I'm a novice when it comes to regular expressions and would appreciate a walkthrough on how you came up with that. — Streamer, Mar 06 '17 at 07:28
Wiktor - Same as above, could you walk me through how you come up with that and why you believe it is the ideal solution — Streamer, Mar 06 '17 at 07:30

score 1 · Accepted Answer · edited May 23 '17 at 12:09

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You need to use a negative lookahead to make sure some pattern is not found to the right of the current location:

/!(?!=)/

See the regex demo. Here, ! is only matched if there is no = immediately after it.

See more about how negative lookahead works here.

edited May 23 '17 at 12:09

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answered Mar 06 '17 at 07:31

Wiktor Stribiżew

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score 1 · Answer 2 · edited May 23 '17 at 11:46

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From what I understand you want to search for a character ! which is NOT followed by a character =. That sounds similar to this question. I tried here:

// regex (?=!(?!\=))

!= ! // matches
foo ! bar // matches
foo != bar // doesn't match

The keyword here is lookaround. See details there (and in the question I linked).

edited May 23 '17 at 11:46

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answered Mar 06 '17 at 07:33

crusy

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Building a regular expression that tests true for '!=' but false for '!'

2 Answers2