48

I'm banging my head against a wall with a regular expression. I'm trying to define an expression that excludes exactly this text 'System' (case insensitive), but can contain the word 'System' providing it's not just that.

Examples:

  • System == INVALID
  • SYSTEM == INVALID
  • system == INVALID
  • syStEm == INVALID
  • asd SysTem == Valid
  • asd System asd == Valid
  • System asd == Valid
  • asd System == Valid
  • asd == Valid
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4 Answers 4

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97

Try this:

^(?!system$)

Or this to match the whole line:

^(?!system$).*$

The regex has a negative look-ahead on its beginning, which doesn't match if "system" is the entire string.

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6 Comments

Just out of curiosity, how does the ?! operator work? I've never used that one (though I've used pretty much regex I tend to find solutions which don't use 'not').
@Alxandr - it checks what follows your current position. For example, c(?!4) will match c from Doc12, but not the c on Doc42. A look-around doesn't capture, so I don't have to worry about replacing the digit, or check edge case (for example, if c were the last character: Doc).
If you don't care about matching the whole line (if you only care about it matching the pattern or not), adding .*$ doesn't make sense, right?
@x-yuri - Right, that's the first option. I you're just filtering string, for example, .*$ is not needed.
I'm trying this in a sql engine and it gives "invalid perl operator" - is there a PCRE equivalent?
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5

Reject if it matches ^system$ (make sure i flag is ON).

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2 Comments

This should be the first option, or course. I naturally (and possibly wrongly) assumed the OP cannot do it.
I can't use this because I'm using a system that uses rules and the rule has to match a regex, I don't have the option to say apply the rule if the regex doesn't match.
1 
^$|^.{1-5}$|.{7}|^[^s]|^.[^y]|^..[^s]|^...[^t]|[^e].$|[^m]$

But use amarghosh's answer if you can.

(updated as per suggestion below)

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3 Comments

heh, I wrote that without testing it. I'm kind of shocked that it actually worked first try. The first two clauses are redundant though, so: (^.{1-6}$)|(.{8})|(^[^s])|(^.[^y])|(^..[^s])|(^...[^t])|([^e].$)|([^m]$) Nice tip about ?! though - I hadn't heard of it.
oops. that regex lets through "systuem". (and fails on empty string, which is apparently acceptable?) try: ^$|^.{1-5}$|.{7}|^[^s]|^.[^y]|^..[^s]|^...[^t]|[^e].$|[^m]$
Steve - Welcome to stack overflow. It is best to update your answer than to add another one as a comment. Also, start the regex line with 4 spaces - it will format it as code.
-1

To match all the words of COMMON_LOGGER except for COMMON_LOGGER. (i.e., not followed by a dot), you can use the negative lookahead assertion (?!.) to exclude any matches that are immediately followed by a dot. Here's an example regex: c6290d3a-e1c4-41f5-b45b-6afdb1cfe3b2

COMMON_LOGGER(?!\.)
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1 Comment

How is this an answer to the (13 year old, badly written) question?

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