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How does this Java regex detect palindromes?

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问题 This is the third part in a series of educational regex articles. It follows How does this regex find triangular numbers? (where nested references is first introduced) and How can we match a^n b^n with Java regex? (where the lookahead "counting" mechanism is further elaborated upon). This part introduces a specific form of nested assertion, which when combined with nested references allows Java regex to match what most people believe is "impossible": palindromes!! The language of palindromes

How does the regular expression ‘(?<=#)[^#]+(?=#)’ work?

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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

How does the regular expression ‘(?<=#)[^#]+(?=#)’ work?

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问题 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

How can we match a^n b^n with Java regex?

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问题 This is the second part of a series of educational regex articles. It shows how lookaheads and nested references can be used to match the non-regular languge a n b n . Nested references are first introduced in: How does this regex find triangular numbers? One of the archetypal non-regular languages is: L = { a n b n : n > 0 } This is the language of all non-empty strings consisting of some number of a \'s followed by an equal number of b \'s. Examples of strings in this language are ab , aabb

Regex lookahead, lookbehind and atomic groups

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问题 I found these things in my regex body but I haven\'t got a clue what I can use them for. Does somebody have examples so I can try to understand how they work? (?!) - negative lookahead (?=) - positive lookahead (?<=) - positive lookbehind (?<!) - negative lookbehind (?>) - atomic group 回答1: Examples Given the string foobarbarfoo : bar(?=bar) finds the 1st bar ("bar" which has "bar" after it) bar(?!bar) finds the 2nd bar ("bar" which does not have "bar" after it) (?<=foo)bar finds the 1st bar