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Get the Opposite of a Regular Expression [duplicate]

Is it possible to write a regex that returns the converse of a desired result? Regexes are usually inclusive - finding matches. I want to be able to transform a regex into its opposite - asserting that there are no matches. Is this possible? If so, how?
http://zijab.blogspot.com/2008/09/finding-opposite-of-regular-expression.html states that you should bracket your regex with
/^((?!^ MYREGEX ).)*$/
, but this doesn't seem to work. If I have regex
/[a|b]./
, the string "abc" returns false with both my regex and the converse suggested by zijab,
/^((?!^[a|b].).)*$/
. Is it possible to write a regex's converse, or am I thinking incorrectly?

Couldn't you just check to see if there are no matches? I don't know what language you are using, but how about this pseudocode?
if (!'Some String'.match(someRegularExpression))
// do something...
If you can only change the regex, then the one you got from your link should work:
/^((?!REGULAR_EXPRESSION_HERE).)*$/

The reason your inverted regex isn't working is because of the '^' inside the negative lookahead:
/^((?!^[ab].).)*$/
^ # WRONG
Maybe it's different in vim, but in every regex flavor I'm familiar with, the caret matches the beginning of the string (or the beginning of a line in multiline mode). But I think that was just a typo in the blog entry.
You also need to take into account the semantics of the regex tool you're using. For example, in Perl, this is true:
"abc" =~ /[ab]./
But in Java, this isn't:
"abc".matches("[ab].")
That's because the regex passed to the matches() method is implicitly anchored at both ends (i.e., /^[ab].$/).
Taking the more common, Perl semantics, /[ab]./ means the target string contains a sequence consisting of an 'a' or 'b' followed by at least one (non-line separator) character. In other words, at ANY point, the condition is TRUE. The inverse of that statement is, at EVERY point the condition is FALSE. That means, before you consume each character, you perform a negative lookahead to confirm that the character isn't the beginning of a matching sequence:
(?![ab].).
And you have to examine every character, so the regex has to be anchored at both ends:
/^(?:(?![ab].).)*$/
That's the general idea, but I don't think it's possible to invert every regex--not when the original regexes can include positive and negative lookarounds, reluctant and possessive quantifiers, and who-knows-what.

You can invert the character set by writing a ^ at the start ([^…]). So the opposite expression of [ab] (match either a or b) is [^ab] (match neither a nor b).
But the more complex your expression gets, the more complex is the complementary expression too. An example:
You want to match the literal foo. An expression, that does match anything else but a string that contains foo would have to match either
any string that’s shorter than foo (^.{0,2}$), or
any three characters long string that’s not foo (^([^f]..|f[^o].|fo[^o])$), or
any longer string that does not contain foo.
All together this may work:
^[^fo]*(f+($|[^o]|o($|[^fo]*)))*$
But note: This does only apply to foo.

You can also do this (in python) by using re.split, and splitting based on your regular expression, thus returning all the parts that don't match the regex, how to find the converse of a regex

In perl you can anti-match with $string !~ /regex/;.

With grep, you can use --invert-match or -v.

Java Regexps have an interesting way of doing this (can test here) where you can create a greedy optional match for the string you want, and then match data after it. If the greedy match fails, it's optional so it doesn't matter, if it succeeds, it needs some extra data to match the second expression and so fails.
It looks counter-intuitive, but works.
Eg (foo)?+.+ matches bar, foox and xfoo but won't match foo (or an empty string).
It might be possible in other dialects, but couldn't get it to work myself (they seem more willing to backtrack if the second match fails?)

Regex in javascript to match either a url pattern or given string

I need a regex expression that will match either a url pattern or a given string [MY_NAME]
Separately both of them are pretty straight forward:
/^(http|https):\/\/[^ "]+$/
/\[MY_NAME\]/
but I cannot combine them together in a single regex expression

You need to use a pipe |:
/^(?:(http|https):\/\/[^\s"]*|\[MY_NAME\])+$/
I added parenthesis and an anti-selector ?: (This is assuming you want to match either string exactly)
Also I added a * to help your regex match correctly (as-well as a couple of other minor changes)

Matching Regex till a character

This is my string:
<address>tel+1234567890</address>
This is my regex:
([\d].*<)
which matches this:
1234567890<
but I dont want to match the last <character.

You can use a positive lookahead:
\d+(?=<)
The (?=...) syntax makes sure what's inside the parens matches at that position, without moving the match cursor forward, thus without consuming the input string. It's also called a zero-width assertion.
By the way, the square brackets in [\d] are redundant, so you can omit them. Also, I've changed the regex, but perhaps you really meant to match this:
\d.*?(?=<)
This pattern matches everything between a digit and a <, including the digit. It makes use of an ungreedy quantifier (*?) to match up until the first < if there are several.

([\d]+)
This should work , try it out and let me know
Check the demo
Also as #LucasTrzesniewski said , you can use the look ahead
(\d+.(?=<))
Here is the demo

How to write a RegEx to check for a certain, specific number of characters?

I am trying to test a string for a state code, the regex I have is
^A[LKSZRAEP]|C[AOT]|D[EC]|F[LM]|G[AU]|HI|I[ADLN]|K[SY]|LA|M[ADEHINOPST]|N[CDEHJMVY]|O[HKR]|P[ARW]|RI|S[CD]|T[NX]|UT|V[AIT]|W[AIVY]$
The issue is, if I have something like "CTA12" as a test string, it will get a match of CT. How can I modify my regex to make it only match state codes that are not part of a larger string?

Your use of anchors with alternation is incorrect, ^AB|DC$ means "strings that start with AB or end with DC". To get the ^ and $ to both apply to each element of the alternation, you need to put the alternation in a group, for example ^(AB|DC)$.
Try changing your regex to the following:
^(A[LKSZRAEP]|C[AOT]|D[EC]|F[LM]|G[AU]|HI|I[ADLN]|K[SY]|LA|M[ADEHINOPST]|N[CDEHJMVY]|O[HKR]|P[ARW]|RI|S[CD]|T[NX]|UT|V[AIT]|W[AIVY])$
The alternative to using a group is to put the ^ and $ as a part of each element in the alternation, for example ^AB$|^DC$, but that would make your regex significantly longer so a group is the way to go.

Nice way to do this regex substitution

I'm writing a javascript function which takes a regex and some elements against which it matches the regex against the name attribute.
Let's say i'm passed this regex
/cmw_step_attributes\]\[\d*\]/
and a string that is structured like this
"foo[bar][]chicken[123][cmw_step_attributes][456][name]"
where all the numbers could vary, or be missing. I want to match the regex against the string in order to swap out the 456 for another number (which will vary), eg 789. So, i want to end up with
"foo[bar][]chicken[123][cmw_step_attributes][789][name]"
The regex will match the string, but i can't swap out the whole regex for 789 as that will wipe out the "[cmw_step_attributes][" bit. There must be a clean and simple way to do this but i can't get my head round it. Any ideas?
thanks, max

Capture the first part and put it back into the string.
.replace(/(cmw_step_attributes\]\[)\d*/, '$1789');
// note I removed the closing ] from the end - quantifiers are greedy so all numbers are selected
// alternatively:
.replace(/cmw_step_attributes\]\[\d*\]/, 'cmw_step_attributes][789]')

Either literally rewrite part that must remain the same in replacement string, or place it inside capturing brackets and reference it in replace.

See answer on: Regular Expression to match outer brackets.
Regular expressions are the wrong tool for the job because you are dealing with nested structures, i.e. recursion.

Have you tried:
var str = 'foo[bar][]chicken[123][cmw_step_attributes][456][name]';
str.replace(/cmw_step_attributes\]\[\d*?\]/gi, 'cmw_step_attributes][XXX]');