What Is a Regular Expression (Regex)?
A regular expression (regex) is a sequence of characters that forms a search pattern. In software engineering, regular expressions are essential for validating form inputs (such as emails or telephone numbers), extracting structured substrings from logs, and performing complex text search-and-replace operations.
Common Regular Expression Flags Explained
g(Global): Searches for all matches throughout the text rather than stopping after the first occurrence.i(Case-Insensitive): Ignores uppercase vs. lowercase distinctions during matching.m(Multiline): Treats the beginning (^) and end ($) anchors as matching the start and end of each individual line, rather than the entire input string.s(DotAll): Allows the wildcard dot (.) to match newline characters as well.u(Unicode): Enables full Unicode pattern matching and surrogate pair support.
Essential Regex Cheatsheet
\d: Matches any digit (equivalent to[0-9]).\w: Matches any word character (alphanumeric plus underscore).\s: Matches whitespace characters (spaces, tabs, newlines).+: Matches one or more repetitions of the preceding token.*: Matches zero or more repetitions.?: Matches zero or one repetition (makes matches optional or lazy).[abc]: Character class matching any character inside the brackets.(abc): Capturing group.
Security Reminder: Catastrophic Backtracking (ReDoS)
Be cautious when nesting quantifiers like (a+)+ against adversarial inputs. Complex, non-deterministic regular expressions can cause exponential execution times known as Regular Expression Denial of Service (ReDoS). Always test your patterns against varied edge-case inputs.