Bắt đầu và kết thúc: `^` và `$`

Anchors are zero-width assertions: they match a position in the string, not a character. The two most basic ones are ^ (start of string) and $ (end of string).

Pattern: ^Errore
Sample:  Errore: connessione persa
         ^^^^^^

Pattern: \.$
Sample:  Frase finale.
                     ^

^Errore matches the word Errore only if it is at the absolute start of the sample. \.$ matches a period only if it is the last character of the sample. Nothing more, nothing less.

Full validation

The most common idiom is to use both together: ^pattern$ matches only if the entire string corresponds to pattern. It is the basis of validation:

^\d{4}$    a string of exactly 4 digits, nothing before, nothing after

Structured validation with start and end

Anchoring a pattern with ^ and $ is essential for validation. Without anchors, the regex \\d{5} will match inside abc1234567def, whereas with ^\\d{5}$ the string is rejected because the entire text must contain exactly 5 digits and nothing else.

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Review exercise

Additional challenge