This guide explains the concepts and practical checks behind the related Tolvix utility. Examples are illustrative; review the result in the service or platform you actually use.
Start with a small pattern
For example, the pattern \\bcat\\b looks for the word cat at a word boundary. It can match cat in a sentence but not the cat portion of concatenate. This is a useful illustration, not a complete validation rule for every possible word or language. Prefer readable patterns and explain complex ones with comments in your code.
Understand flags
The i flag makes matching case-insensitive. The g flag finds multiple matches. The m flag changes how start and end anchors behave across lines. The s flag lets a dot match line terminators. The u flag enables Unicode-aware behavior. Some newer flags depend on browser support. Avoid adding flags you do not need.
Test edge cases and captures
Test ordinary examples, empty strings, punctuation, newlines, repeated matches and unexpected characters. Capturing parentheses return matched groups that can be used for extraction. Non-capturing groups (?:...) group parts without adding a capture. When using a global expression with repeated exec calls, be aware that lastIndex changes between calls.
Worked example: validate a simple code
To find codes made from two uppercase letters followed by four digits, a JavaScript pattern could be \b[A-Z]{2}\d{4}\b. Test it with a positive example such as AB1234 and negative examples such as A1234, ABC1234 and ab1234. The last example should fail unless you choose case-insensitive matching.
This pattern only checks the shape of the text. It does not confirm that a code exists in a database or belongs to a real person or product. Add boundary cases and test the exact expression in the application that will use it.