Regular Expression Syntax and Metacharacter Guide

Regular expressions are defined between delimiter characters, typically slashes (/), where the content inside the delimiters represents the pattern to be matched against a target string.

Common metacharacters include + , * , and ? . The + metacharacter requires its preceding character to appear one or more times, * allows zero or more occurrences, and ? permits zero or one occurrence. Examples: /fo+/ matches "fo", "fool", "football", etc.; /eg*/ matches "e", "eg", "egg", "easy", etc.; /Wil?/ matches "Wi" or "Wil".

Additional metacharacters provide shortcuts for common character sets: \s matches any whitespace (space, tab, newline); \S matches any non‑whitespace; \d matches digits 0‑9; \w matches letters, digits, or underscore; \W matches any character not matched by \w ; . matches any character except newline. Example patterns: /\s+/ matches one or more whitespace characters; /\d000/ matches a digit followed by three zeros, useful for locating amounts of a thousand in financial data.

Quantifiers can also specify exact repetition ranges using curly braces. For instance, /jim{2,6}/ requires the character m to appear between two and six times, matching strings like "jimmy" or "jimmmmmy".

Anchors (positioning characters) restrict where a pattern can appear: ^ asserts the start of a string, $ asserts the end, \b asserts a word boundary, and \B asserts a non‑boundary. Examples: /^hell/ matches strings beginning with "hell"; /ar$/ matches strings ending with "ar"; /\bbom/ matches "bomb" or "bom" at the start of a word; /man\b/ matches "man" at the end of a word.

Character classes allow matching any character within a set: /[A-Z]/ matches any uppercase letter, /[a-z]/ any lowercase letter, /[0-9]/ any digit, and combined groups like /([a-z][A-Z][0-9])+ / match strings containing a lowercase letter, an uppercase letter, and a digit in sequence.

Logical operators enable alternative matches: the pipe | works as OR, e.g., /to|too|2/ matches "to", "too", or "2". The negation operator [^] inside brackets excludes characters, as in /[^A-C]/ , which matches any character except A, B, or C.

When a metacharacter needs to be matched literally, it must be escaped with a backslash. For example, /Th\*/ matches the literal string "Th*" rather than interpreting * as a quantifier.