Regex Alternative Groups: Alternation and Group Capture

Alternation is first-match, not longest-match

The engine tries alternatives left to right and takes the first that allows the overall match to succeed — not the longest:

/cat|catalog/    on "catalog"   ->  matches "cat"
/catalog|cat/    on "catalog"   ->  matches "catalog"

Put longer or more specific alternatives first. This differs from POSIX engines such as grep, which use leftmost-longest — the same pattern can behave differently in grep -E and in Python.

Scope the alternation

The pipe has the lowest precedence, so it splits the entire pattern unless grouped:

/^cat|dog$/       # "starts with cat" OR "ends with dog"
/^(cat|dog)$/     # exactly "cat" or exactly "dog"
/^(?:cat|dog)$/   # same, without capturing

The first pattern is a very common validation bug — it accepts "catfish" and "hotdog".

A quantified group keeps only the last capture

(\w+,?)+ applied to a,b,c leaves group 1 holding only c. The group captured three times and each capture overwrote the previous one.

To collect every occurrence, match repeatedly instead of quantifying the group — matchAll in JavaScript, findall or finditer in Python, preg_match_all in PHP:

[...'a,b,c'.matchAll(/(\w+)/g)].map((m) => m[1]);   // ['a','b','c']

.NET is the exception: Group.Captures preserves every capture of a repeated group, which is occasionally exactly what you need and is not portable.

Character classes beat alternation

For single characters, a class is faster and clearer: [abc] rather than (?:a|b|c). The engine checks membership instead of trying three branches with backtracking.

Reserve alternation for multi-character alternatives, and factor out common prefixes where you can — re(?:ad|d) does less work than read|red.