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 chars.
They are called lookarounds; they allow you to assert if a pattern matches or not, without actually making the match. There are 4 basic lookarounds:
- Positive lookarounds: see if we CAN match the
pattern...(?=pattern)- ... to the right of current position (look ahead)(?<=pattern)- ... to the left of current position (look behind)
- Negative lookarounds - see if we can NOT match the
pattern(?!pattern)- ... to the right(?<!pattern)- ... to the left
As an easy reminder, for a lookaround:
=is positive,!is negative<is look behind, otherwise it's look ahead
References
But why use lookarounds?
One might argue that lookarounds in the pattern above aren't necessary, and #([^#]+)# will do the job just fine (extracting the string captured by \1 to get the non-#).
Not quite. The difference is that since a lookaround doesn't match the #, it can be "used" again by the next attempt to find a match. Simplistically speaking, lookarounds allow "matches" to overlap.
Consider the following input string:
and #one# and #two# and #three#four#
Now, #([a-z]+)# will give the following matches (as seen on rubular.com):
and #one# and #two# and #three#four#
\___/ \___/ \_____/
Compare this with (?<=#)[a-z]+(?=#), which matches:
and #one# and #two# and #three#four#
\_/ \_/ \___/ \__/
Unfortunately this can't be demonstrated on rubular.com, since it doesn't support lookbehind. However, it does support lookahead, so we can do something similar with #([a-z]+)(?=#), which matches (as seen on rubular.com):
and #one# and #two# and #three#four#
\__/ \__/ \____/\___/
References
As another poster mentioned, these are lookarounds, special constructs for changing what gets matched and when. This says:
(?<=#) match but don't capture, the string `#`
when followed by the next expression
[^#]+ one or more characters that are not `#`, and
(?=#) match but don't capture, the string `#`
when preceded by the last expression
So this will match all the characters in between two #s.
Lookaheads and lookbehinds are very useful in many cases. Consider, for example, the rule "match all bs not followed by an a." Your first attempt might be something like b[^a], but that's not right: this will also match the bu in bus or the bo in boy, but you only wanted the b. And it won't match the b in cab, even though that's not followed by an a, because there are no more characters to match.
To do that correctly, you need a lookahead: b(?!a). This says "match a b but don't match an a afterwards, and don't make that part of the match". Thus it'll match just the b in bolo, which is what you want; likewise it'll match the b in cab.
They're called look-arounds: http://www.regular-expressions.info/lookaround.html
来源：https://stackoverflow.com/questions/3092797/how-does-the-regular-expression-work