Question 1

Which regex flavour does this use?

Accepted Answer

JavaScript's own, running in your browser, so what you see here is exactly what your code will do in a browser or in Node. It is not PCRE: JavaScript has no atomic groups, no possessive quantifiers and no recursion, and lookbehind is supported in current browsers but was added late. Patterns copied from PHP, Python or Perl mostly work, but not always.

Question 2

Why does my pattern only find the first match?

Accepted Answer

Because the g flag is off. Without it, a regex stops at the first match, which is what you want for validation and not what you want for search and replace. The same flag is what makes replace change every occurrence rather than one.

Question 3

What does the empty match marker mean?

Accepted Answer

That your pattern can match nothing at all, usually because everything in it is optional: a*, \d?, or an alternation with an empty branch. Run with the g flag and it matches the empty string between every character, which is almost never what was intended and is a common cause of loops that never end.

Question 4

Why did my pattern get stopped for taking too long?

Accepted Answer

Because it was still backtracking after the time limit. The usual cause is a quantifier inside a quantifier, such as (a+)+ or (\w+\s?)*, where the engine has an exponential number of ways to split the same text and tries them all before giving up. On a real server that is a denial of service; the fix is to make the inner part unable to match what the outer part already can.

Question 5

What is the difference between a numbered and a named group?

Accepted Answer

Only how you refer to it. (\d{4}) is group 1, while (?<year>\d{4}) is both group 1 and "year". Names survive edits that renumber the groups, so a pattern that gets a new capture in the middle does not silently break the code reading $2. Both are listed under each match here.

Question 6

Does the replacement support anything besides $1?

Accepted Answer

Yes: $& inserts the whole match, $<name> a named group, $` and $' the text before and after the match, and $$ a literal dollar sign. A dollar followed by anything else is left alone, which is why an unescaped price in a replacement usually comes through unharmed.

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