lookbehind

Can you use backreferences in a lookbehind? Let's say I want to split wherever behind me a character is repeated twice. String REGEX1 = "(?<=(.)\\1)"; // DOESN'T WORK! String REGEX2 = "(?<=(?=(.)\\1)..)"; // WORKS! System.out.println(java.util.Arrays.toString( "Bazooka killed the poor aardvark (yummy!)" .split(REGEX2) )); // prints "[Bazoo, ka kill, ed the poo, r aa, rdvark (yumm, y!)]" Using REGEX2 (where the backreference is in a lookahead nested inside a lookbehind) works, but REGEX1 gives this error at run-time: Look-behind group does not have an obvious maximum length near index 8 (?<=(.)

I am trying to use lookbehinds in a regular expression and it doesn't seem to work as I expected. So, this is not my real usage, but to simplify I will put an example. Imagine I want to match "example" on a string that says "this is an example". So, according to my understanding of lookbehinds this should work: (?<=this\sis\san\s*?)example What this should do is find "this is an", then space characters and finally match the word "example". Now, it doesn't work and I don't understand why, is it impossible to use '+' or '*' inside lookbehinds? I also tried those two and they work correctly, but

There are some features in modern regex engines which allow you to match languages that couldn't be matched without that feature. For example the following regex using back references matches the language of all strings that consist of a word that repeats itself: (.+)\1 . This language is not regular and can't be matched by a regex that does not use back references. Does lookaround also affect which languages can be matched by a regular expression? I.e. are there any languages that can be matched using lookaround that couldn't be matched otherwise? If so, is this true for all flavors of

Is there a way to do negative and positive lookbehind in VBA regex? I want to not match if the string starts with "A", so I am currently doing ^A at the start of the pattern, then removing the first character of match(0). Obviously not the best method! I am using the regExp object. brettdj VBA offers positive and negative lookaheads but rather inconsistently not lookbehind. The best example of using Regex with VBA that I have seen is this article by Patrick Matthews [Updated example using Execute rather than Replace ] While I am not completely clear on your usage you could use a function like

For example,the regex below will cause failure reporting lookbehind assertion is not fixed length : #(?<!(?:(?:src)|(?:href))=["\']?)((?:https?|ftp)://[^\s\'"<>()]+)#S Such kind of restriction doesn't exist for lookahead . Lookahead and lookbehind aren't nearly as similar as their names imply. The lookahead expression works exactly the same as it would if it were a standalone regex, except it's anchored at the current match position and it doesn't consume what it matches. Lookbehind is a whole different story. Starting at the current match position, it steps backward through the text one

I have the following regex in a C# program, and have difficulties understanding it: (?<=#)[^#]+(?=#) I'll break it down to what I think I understood: (?<=#) a group, matching a hash. what's `?<=`? [^#]+ one or more non-hashes (used to achieve non-greediness) (?=#) another group, matching a hash. what's the `?=`? So the problem I have is the ?<= and ?< part. From reading MSDN, ?<name> is used for naming groups, but in this case the angle bracket is never closed. I couldn't find ?= in the docs, and searching for it is really difficult, because search engines will mostly ignore those special