What each regex flag does

Flags change how the whole pattern is applied. In the regex tools they are toggle buttons under the pattern box.

• g — global. Find every match rather than stopping at the first. Almost always what you want when testing.
• i — ignore case. Match without regard to upper or lower case.
• m — multiline. ^ and $ match at every line break rather than only at the ends of the string.
• s — dotAll. Lets . match newline characters, which it otherwise will not.
• u — unicode. Treats the pattern as code points and enables \p{...} property escapes.
• y — sticky. Matches only from the current position, with no scanning ahead.
• d — indices. Records the start and end offset of every capture group.

The two that surprise people

m does not make . match newlines — that is s. And g changes the behaviour of repeated calls in JavaScript because the expression keeps a lastIndex between them, which is a common source of a regex that works once and then fails.

In the tools

The tester adds g automatically when finding matches, so the table shows all of them. Every flag button has a tooltip describing what it does.