nested-reference

Why does Java regex engine throw StringIndexOutOfBoundsException on a + repetition?

由 南楼画角 提交于 2019-12-18 12:54:24
问题 I've written a regex pattern to find Fibonacci numbers (it doesn't matter why, I just did). It works wonderfully as expected (see on ideone.com): String FIBONACCI = "(?x) .{0,2} | (?: (?=(\\2?)) (?=(\\2\\3|^.)) (?=(\\1)) \\2)++ . "; for (int n = 0; n < 1000; n++) { String s = new String(new char[n]); if (s.matches(FIBONACCI)) { System.out.print(n + " "); } } // 0 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 A possessive repetition (i.e. ++ on the main "loop") is crucial, because you don't

How does this regex replacement reverse a string?

由 白昼怎懂夜的黑 提交于 2019-11-29 11:38:02
问题 This is the fourth part in a series of educational regex articles. It show how the combination of nested reference (see: How does this regex find triangular numbers?) to "count" within assertions (see: How can we match a^n b^n with Java regex?) can be used to reverse a string. The programmatically generated pattern uses meta-pattern abstractions (see: How does this Java regex detect palindromes?). For the first time in the series, these techniques are used for replacement instead of whole

How does this PCRE pattern detect palindromes?

由 耗尽温柔 提交于 2019-11-27 21:16:46
This question is an educational demonstration of the usage of lookahead, nested reference, and conditionals in a PCRE pattern to match ALL palindromes, including the ones that can't be matched by the recursive pattern given in the PCRE man page. Examine this PCRE pattern in PHP snippet: $palindrome = '/(?x) ^ (?: (.) (?= .* ( \1 (?(2) \2 | ) ) $ ) )* .? \2? $ /'; This pattern seems to detect palindromes, as seen in this test cases ( see also on ideone.com ): $tests = array( # palindromes '', 'a', 'aa', 'aaa', 'aba', 'aaaa', 'abba', 'aaaaa', 'abcba', 'ababa', # non-palindromes 'aab', 'abab',

How does this regex find triangular numbers?

由 与世无争的帅哥 提交于 2019-11-27 17:10:27
Part of a series of educational regex articles, this is a gentle introduction to the concept of nested references. The first few triangular numbers are: 1 = 1 3 = 1 + 2 6 = 1 + 2 + 3 10 = 1 + 2 + 3 + 4 15 = 1 + 2 + 3 + 4 + 5 There are many ways to check if a number is triangular. There's this interesting technique that uses regular expressions as follows: Given n , we first create a string of length n filled with the same character We then match this string against the pattern ^(\1.|^.)+$ n is triangular if and only if this pattern matches the string Here are some snippets to show that this

How does this Java regex detect palindromes?

由 江枫思渺然 提交于 2019-11-27 13:34:26
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 is non- regular ; it's actually context-free (for a given alphabet). That said, modern regex

How does this PCRE pattern detect palindromes?

由 我是研究僧i 提交于 2019-11-26 20:35:15
问题 This question is an educational demonstration of the usage of lookahead, nested reference, and conditionals in a PCRE pattern to match ALL palindromes, including the ones that can't be matched by the recursive pattern given in the PCRE man page. Examine this PCRE pattern in PHP snippet: $palindrome = '/(?x) ^ (?: (.) (?= .* ( \1 (?(2) \2 | ) ) $ ) )* .? \2? $ /'; This pattern seems to detect palindromes, as seen in this test cases (see also on ideone.com): $tests = array( # palindromes '', 'a

How does this regex find triangular numbers?

由 泄露秘密 提交于 2019-11-26 18:55:47
问题 Part of a series of educational regex articles, this is a gentle introduction to the concept of nested references. The first few triangular numbers are: 1 = 1 3 = 1 + 2 6 = 1 + 2 + 3 10 = 1 + 2 + 3 + 4 15 = 1 + 2 + 3 + 4 + 5 There are many ways to check if a number is triangular. There's this interesting technique that uses regular expressions as follows: Given n , we first create a string of length n filled with the same character We then match this string against the pattern ^(\1.|^.)+$ n

How does this Java regex detect palindromes?

由 房东的猫 提交于 2019-11-26 16:25:11
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 can we match a^n b^n with Java regex?

由 て烟熏妆下的殇ゞ 提交于 2019-11-25 23:48:10
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
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