ASCII 91 — [

The left square bracket opens array subscript expressions, indexing operations, and character class definitions across computing. In most languages, 'array[index]' accesses elements through bracket notation. In regular expressions, '[...]' defines a character class matching any single character listed within. In JSON, '[' begins an array literal. In Markdown, '[text](url)' creates hyperlinks. In Python, brackets enable powerful list slicing with 'list[start:stop:step]' syntax. Bracket matching is a fundamental feature of code editors and a classic data structure interview problem.

Punctuation marks serve as the structural scaffolding of written language, delineating sentences, clauses, and phrases while conveying emphasis and grouping. In programming, these same characters gain powerful additional roles as operators, statement terminators, delimiters, and syntactic markers that compilers and interpreters rely upon. This dual significance in both natural language and code syntax means punctuation characters carry outsized functional importance relative to their small count in the ASCII character set.

In the ASCII encoding table, Left Square Bracket is assigned code point 91 in decimal (0x5B hexadecimal, 133 octal, 01011011 binary). The 7-bit ASCII standard, first published in 1963 by the American Standards Association, defines exactly 128 characters that remain the foundation of text encoding systems worldwide. UTF-8, the dominant encoding on the modern web, is fully backward compatible with ASCII — every ASCII character is encoded as the identical single byte in UTF-8, guaranteeing that Left Square Bracket works reliably across all operating systems, programming languages, and internet protocols.