regular expressions

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The true power of regular expressions (2012)
5 hours ago
- Regular expressions in programming are far more powerful than the formal 'regular languages' theory suggests, capable of matching context-free and even some context-sensitive languages.
- The Chomsky hierarchy categorizes formal languages, and modern regex implementations like PCRE can handle languages beyond regular ones, including those with recursive patterns.
- Using recursive subpatterns (e.g., (?1)) allows regex to match context-free languages like {a^n b^n}.
- Grammar definitions can be directly translated into regex using DEFINE and named subpatterns, as shown with an RFC 5322 email regex.
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Pg_re2: Postgres extension for fast, RE2-powered regular expressions in Postgres
25 days ago
- ClickHouse introduces pg_re2, a Postgres extension for fast RE2-powered regular expressions, emphasizing speed and compatibility.
- RE2 uses finite automata for faster, more predictable performance compared to Postgres's backtracking POSIX regex, with benchmarks showing 1.8x to 8.6x speedups.
- pg_re2 adds index support (@~ operator) with btree and GIN indexes, providing 1.1x to 1.8x acceleration and better trigram handling for patterns.
- Compatibility with ClickHouse regex functions is key, as pg_re2 addresses syntax differences (e.g., \b meaning word boundary vs. backspace) to ensure consistent results when using pg_clickhouse.
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A few thought about snarky answers on StackOverflow (2019)
a month ago
- Regular expressions can identify individual syntax units in XHTML such as comments, start tags, and end tags, but they cannot match start tags to end tags arbitrarily due to lack of recursion.
- A common misunderstanding is that HTML is a Type 2 grammar and regex is Type 3, making matching impossible, but regex can be used with a counter or stack for parsing to handle nesting.
- Parsing involves tokenization with regex and building a tree structure, but standard parsers like DOM do not expose distinctions like empty elements vs. self-closing tags, limiting solutions to the original problem.
- StackOverflow's voting system sometimes promotes memes and misinformation, as seen in the snarky answers about regex and XHTML, overshadowing legitimate solutions.

regular expressions

The true power of regular expressions (2012)

Pg_re2: Postgres extension for fast, RE2-powered regular expressions in Postgres

A few thought about snarky answers on StackOverflow (2019)