Regex Tester

A greedy quantifier takes as much as it can and gives characters back only when the rest of the pattern fails; a lazy one, written with a question mark after it, takes as little as it can and adds more only when it has to. It is why <.*> matches an entire line of HTML from the first tag to the last, while <.*?> matches one tag. Every quantifier here says which it is, because it is the single most common source of a pattern that almost works.

When a quantifier is applied to something already quantified, such as (a+)+, the engine can try an exponential number of ways to divide the input. On a string that nearly matches but does not, a pattern like that will hang rather than return, which is the mechanism behind most regular expression denial of service. The classic shapes are flagged here, though the general question cannot be settled by inspection, so a pattern that passes can still be slow.

Both group things together so a quantifier or an alternation applies to the whole lot. A capturing group also remembers what it matched, so you can refer to it as $1 in a replacement or as \1 later in the pattern. A non-capturing group, written (?:...), does the grouping without the bookkeeping, which keeps the numbering of the groups you do care about from shifting every time you add a pair of brackets.

Not exactly. This runs JavaScript's own engine, so what you see is precisely what your JavaScript will do. Python, PCRE, Go and Java differ on lookbehind, on what a word boundary means outside ASCII, on named group syntax and on whether the engine backtracks at all — Go's RE2 does not, which is why it refuses backreferences. A pattern that works here is a good starting point elsewhere rather than a guarantee.

Because almost every email pattern in circulation, including the one offered here, is a pragmatic approximation. The pattern that actually implements RFC 5322 runs to several thousand characters, and it still accepts addresses that no mail server will deliver to, since whether a mailbox exists is not a question syntax can answer. Rejecting anything without an @ and a dot after it catches typos; anything stricter starts rejecting real people.

No. The pattern and the text are matched in your browser and nothing leaves the page, which matters when what you are testing against is a log file, a customer list or anything else you would not paste into a stranger's server.