Regex match any string that isn't preceded by another string

Question

I suck at regex and I've been trying to get through this problem. The problem is that I have a string and I want to match any "+" or "-" that aren't preceded by a "^".

Example string:

100x^-3+2x-10

The string above would be mathematically formatted or read as "one hundred times x to the -3 power, plus two times x, minus ten".

And I want to match the "+" before "2x" and the "-" before the "10" but not the "-" after "100x^". I hope it's not that confusing. I have tried the following reg, with no luck:

[^\^][\+|\-]

Obviously I'm missing a big detail somewhere.

Are you trying to match all the text after the operator or just trying to test if the string contains a plus or minus not preceded by a caret. — Asad Saeeduddin, Mar 17 '13 at 00:01

score 0 · Answer 1 · answered Mar 17 '13 at 00:05

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You could use the old trick of simulating look behind by reversing the string, then matching against /[\+\-](?!=\^)/.

answered Mar 17 '13 at 00:05

Asad Saeeduddin

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yeah this works too i guess i need to study regex a lot more, thanks – user1549913 Mar 17 '13 at 03:58

score 0 · Answer 2 · answered Mar 17 '13 at 00:21

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It runs ok in chrome console, anyway you can try with : "100x^-3+2x-10".match(/[^\^$]([\-|\+])/ig) , I advice you to test your regex in the console of Google Chrome.

answered Mar 17 '13 at 00:21

Daniel Flores

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There is an problem with the character class. The pipe character does not need to be there. – Asad Saeeduddin Mar 17 '13 at 00:36

score 0 · Accepted Answer · answered Mar 17 '13 at 00:41

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If you need to split by + or - :

"100x^-3+2x-10".replace(/([^^])[-+]/g,'$1@').split('@')
Output: ["100x^-3", "2x", "10"]

I used @ as a temp char (but you can use a unique set of chars) to mark the signs not prefixed by ^ then safely split by the chosen temp char.

answered Mar 17 '13 at 00:41

CSᵠ

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thank you, you actually look ahead of what I wanted this is the reason why i posted the question, many thanks! – user1549913 Mar 17 '13 at 03:59

score 0 · Answer 4 · answered Mar 17 '13 at 00:52

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Give this a shot:

/(?<!\^)[+-]/

answered Mar 17 '13 at 00:52

woemler

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im posting this comment just incase somebody finds this question useful, i had said this worked (the reg you gave) because i tried it in RegExr app, although I just tried in my script with match and i got the following error: "Uncaught SyntaxError: Invalid regular expression: /(?<!\^)[+-]/: Invalid group", i don't know why the script can't make it work but my reg testing app does, oh well thanks anyway! – user1549913 Mar 17 '13 at 04:17
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My appologies, it looks like Javascript does not support look-behinds: http://stackoverflow.com/questions/5973669/javascript-regexp-lookbehind-assertion-is-causing-a-invalid-group-error – woemler Mar 17 '13 at 18:38

Regex match any string that isn't preceded by another string

4 Answers4