Lookahead obeys /m just fine, but /m has no impact on the dot (whereas /s does). I think you are a bit confused about what the /m and /s flags actually do (this is a common point of confusion since the flags are named so terribly). Please see http://blog.stevenlevithan.com/archives/singleline-multiline-confusing for the details.

XRegExp("^a(?=.)$", "ms").test("a\n") should and does return true.
XRegExp("^a(?!.)$", "ms").test("a\n") should and does return false.

This is long-established, cross-library-consistent regex behavior (albeit confusing due to the flag names).

Imo, with multiline enabled a lookAhead should ignore the line terminator (and the rest of the expression) if the following statement is a ‘$’.

Thanks for the reply.

I’m using FireFox but anyway, I noticed the same problem in JAVA.
JAVA says:
“When in MULTILINE mode $ matches just before a line terminator or the end of the input sequence” (http://java.sun.com/j2se/1.5.0/docs/api/java/util/regex/Pattern.html#lt)
So when I’m at the end of a line from many lines and have the ‘m’ flag up. I expect, to what java says, that when I preform a lookAhead with ‘.’ from that point _and_ have the ’s’ flag up that it will not see the line terminator in any possible way.

So for now I presume that a lookAhead will not obey the ‘m’ flag and sees the line terminators not as EOL.

Java bug?
Not specified in the RegEx world?
Another flag to introduce to overcome this?
The end of the world?

With your first regex, you have the /s flag turned on, so it’s absolutely correct that only the second line matches (since the dot matches any character including line beaks). Turning off /s allows the regex to match both lines, which again is correct. In IE, turning off /s does not allow the regex to match both lines, but that is because of an IE regex bug (possibly the bug described at http://blog.stevenlevithan.com/archives/regex-lookahead-bug or something similar). You can easily test the difference cross-browser using this stripped down version of your regex: javascript:alert(/\d(?!.)/m.test("1\r"));. In IE it returns false; in other browsers, true.

With your second regex, again, everything is as it should be.

http://regexpal.com/?flags=ms&regex=^((25[0-5]|(2[0-4]|1\d|[1-9]%3F)\d)(\.(%3F%3D.)|)){4}%24&input=1.2.3.5.%0A1.2.3.4%0A

Counts both as valid, yet only the 2nd is valid.