Alternation: `|`

The pipe | is the OR of regex: the pattern matches if one of the alternatives matches. Its precedence is very low: | separates everything to its left and right up to the enclosing group or the start of the pattern.

Pattern: cane|gatto|criceto
Sample:  Ho un cane, un gatto e un criceto.
               ^^^^     ^^^^^         ^^^^^^^

Parentheses drive precedence

Without parentheses, alternatives span from start to end of the pattern. Almost always you want to limit the OR to a portion of the pattern: wrap it in a group.

Pattern   Significato (errato vs corretto)
^a|b$     "inizio + a" oppure "b + fine"           (probabilmente sbagliato)
^(a|b)$   "inizio + (a oppure b) + fine"           (probabilmente giusto)

About the typical ordering of alternatives: put the most specific/longest ones first. The engine tries from left to right and stops at the first one that matches: gat|gatto would always only match gat.

Precedence and isolation of alternatives

The | operator has very low precedence. If you write ^abc|def$, you are looking for "a string starting with abc OR a string ending with def". To look for "start of string followed by abc or def, followed by end of string", you must write ^(abc|def)$.

Try it

Review exercise

Additional challenge