Regex Tester & Debugger

For simple checks, you can match a basic HTML tag with something like <([a-z][a-z0-9]*)\b[^>]*> in case-insensitive mode. That finds an opening tag name and optional attributes. However, regex is not reliable for fully parsing HTML because HTML can contain nested tags, comments, scripts, malformed markup, and quoted > characters. For real extraction or validation, use an HTML parser. Use regex only for lightweight tasks, such as finding likely tags in a snippet, cleaning controlled text, or quick editor searches.

A simple regex for YYYY-MM-DD is ^\d{4}-\d{2}-\d{2}$. This checks the shape, such as 2026-05-06, but it still allows impossible dates like 2026-99-99. A stronger pattern is ^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$, which limits month and day ranges. Even then, it does not fully validate leap years or month-specific days. My usual advice is: use regex to check the format, then use a date library or built-in Date validation for actual calendar correctness.

Paste your test text into the Regex Tester, enter the pattern, and choose JavaScript-style flags such as g for global, i for case-insensitive, and m for multiline. Live highlighting shows each match immediately, which makes it easier to see whether your expression is too broad or too narrow. Add sample text that includes normal cases, edge cases, and things that should not match. If your regex has groups, review the captured values too. This is much faster than guessing inside application code.

A positive lookahead checks that something appears next, without consuming it as part of the match. The syntax is (?=pattern). For example, \w+(?=:) matches a word only when it is followed by a colon, but the colon is not included in the match. Lookaheads are useful for password rules, conditional matching, and finding text before a marker. The key idea is simple: "match this position only if the next characters satisfy this rule." It is powerful, but overusing it can make regex harder to maintain.

In regex, \b means a word boundary. It does not match a visible character; it matches a position between a word character and a non-word character. For example, \bcat\b matches "cat" as a separate word, but not the cat inside "catalog". This is useful for search-and-replace tasks where you only want whole words. One thing to remember: "word character" usually means letters, digits, and underscore in JavaScript regex, so behavior with non-English scripts may require Unicode-aware patterns.

Back-references let you reuse captured parts of a match in the replacement. First put the part you want to keep inside parentheses. For example, with pattern (\w+) (\w+) and replacement $2, $1, the text "Asha Rao" becomes "Rao, Asha" in JavaScript-style replacement. This is great for reformatting names, dates, IDs, or log lines. Always test on sample data first, because one extra capturing group changes the numbering. If you do not need to capture a group, use a non-capturing group like (?:...) instead.

Most of the time, this happens because a quantifier is greedy. For example, .* tries to take as much text as possible while still allowing the pattern to match. If you wanted the shortest match, use a lazy quantifier such as .*? or make the character class more specific, like [^<]* for text before a tag. Anchors also help: ^ starts at the beginning and $ ends at the end. In a tester, add several examples and watch where the highlight starts and stops.