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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.
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A regular expression (shortened as regex or regexp; sometimes referred to as rational expression) is a sequence of characters that specifies a search pattern in text. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation.
$ means "Match the end of the string" (the position after the last character in the string).
Example : The regular expression ab+c will give abc, abbc, abbc, … and so on. The curly braces {…}: It tells the computer to repeat the preceding character (or set of characters) for as many times as the value inside this bracket.
Solution: As we know, any number of a's means a* any number of b's means b*, any number of c's means c*. Since as given in problem statement, b's appear after a's and c's appear after b's. So the regular expression could be: R = a* b* c*
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:
pattern... (?=pattern) - ... to the right of current position (look ahead)(?<=pattern) - ... to the left of current position (look behind)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 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# \__/ \__/ \____/\___/ If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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