名前付きグループと後方参照

When the number of groups grows, remembering that $2 is "the month" and $5 is "the year" becomes fragile. The (?<name>...) syntax lets you name a group: you access it by name via groups.name (and in replacements as $<name>).

Pattern: (?<anno>\d{4})-(?<mese>\d{2})-(?<giorno>\d{2})
Sample:  2024-03-15
Gruppi:
  anno    = "2024"
  mese    = "03"
  giorno  = "15"

The editor shows the named groups next to the match.

Backreference

A backreference lets you refer, inside the same pattern, to a previous capture. The syntax:

• \1, \2, … -- numbered reference (to group 1, 2, …).
• \k<name> -- named reference.

Pattern: \b(\w+)\s+\1\b
Sample:  un test un test il il gatto
                                ^^^^^

It matches consecutive duplicate words: (\w+) captures a word, then \s+\1 requires a space and the same word.

Self-documenting code and named back-references

Named groups improve the readability of JavaScript code: instead of referring to match[1], you access match.groups.name. Inside the pattern itself, referencing them is done with the \\k<name> syntax.

Try it

Review exercise

Additional challenge