Regular expressions (regex) are sequences of characters defining search patterns used to find specific text combinations within files. Unlike simple searches matching exact words or basic wildcards (like *.txt), regex uses special syntax to match complex text structures – like specific character sequences ([0-9] for any digit), repeating patterns (colou?r matches both "color" and "colour"), or positions (^start, end$). This makes searching far more flexible and powerful for text-based files.

In practice, many tools utilize regex for file content searches. Developers use command-line tools like grep (Linux/macOS) or findstr (Windows) to search log files for error patterns, such as grep "error: [A-Z]+-[0-9]+" app.log. Text editors and IDEs (like VS Code, Sublime Text, Notepad++) often support regex within their "Find in Files" features, enabling searches across project directories for complex code patterns, like finding all function calls matching calculate\_\w+\\(.

Regex offers significant advantages for precision text searching in logs, codebases, and documents. However, they have a steep learning curve, can become unreadable for very complex patterns, and are primarily suited for textual content (not binary files). Performance may degrade with huge files or inefficient patterns. Ethical concerns like unintended pattern matches are possible, so testing is crucial. Built-in support in most search tools ensures continued relevance, though GUIs are increasingly simplifying complex regex use.