Regular expressions (regex or regexp) are sequences of characters that define a search pattern. They are used across virtually every programming language and text editor for finding, matching, extracting, and replacing text. A regex like \d{3}-\d{4} matches phone number patterns, while [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} matches email addresses.
Regex patterns are built from literal characters and special metacharacters. A dot (.) matches any single character. A star (*) means "zero or more of the previous element." Square brackets define character classes ([a-z] matches any lowercase letter). Parentheses create capture groups for extracting specific parts of a match. Anchors like ^ and $ match the start and end of a line.
Different programming languages have slightly different regex engines (PCRE in PHP, RE2 in Go, the built-in engine in JavaScript), but the core syntax is nearly universal. Most regex engines support lookaheads, lookbehinds, non-greedy quantifiers, and named capture groups for complex pattern matching.
Regex is one of those skills that pays dividends everywhere. Log parsing, data validation, text extraction, find-and-replace across codebases, URL routing, input sanitization, and web scraping all rely heavily on regular expressions. The learning curve is steep but the productivity gains are enormous, especially for tasks that would otherwise require dozens of lines of procedural string manipulation code.
TerminalFeed offers a Regex Tester tool that lets you write patterns and test them against sample text in real time, with match highlighting and capture group extraction. It is one of the most-used developer tools on the site.