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 re...
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 anbn. Nested references are first introduced in: How does this regex find triangular numbers? One of the archetypal non-regular languages is: L = { anbn: n > 0 } This i...
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 as...
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 a...
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...
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) ^ (?: (.) (?= ...
This is an experimental new feature for StackOverlow: exercising your regex muscles by solving various classical problems. There is no one right answer, and in fact we should collect as many right answers as possible, as long as they offer educational value. All flavors accepted, but please document it clearly. As much as practical, p...