How do I match a repeated character or word with regex quantifiers?

Each quantifier applies to the immediately preceding regex item. For the letter a:

• a? matches zero or one a. It can match an empty string or a.
• a* matches zero or more adjacent a characters. That includes no a, one a, or several, such as aaaa.
• a+ matches one or more adjacent a characters. It requires at least one, so it can match a or aaaa, but not an empty string.

Python’s standard-library re documentation and the JavaScript guide describe these meanings. [S1] [S2]

A key detail: a+ repeats only the a; it does not repeat a whole word or longer sequence unless you group that sequence.

Group the complete piece you want repeated, then put the quantifier after the closing parenthesis. For example, (ha){2} means the group ha appears exactly twice, so it matches haha. Without grouping, ha{2} repeats only the immediately preceding a; it means h followed by two a characters, so it matches haa, not haha. Both references explain this grouping rule. [S1] [S2]

The same idea works for a longer pattern: put parentheses around the entire sequence that should repeat. The quantifier after the closing parenthesis applies to that grouped sequence, rather than just its final character.

For the quantifiers covered here, the notation is the same in Python’s standard-library re module and JavaScript regular expressions: ?, *, +, and {n}. Both references document those meanings and the need to group a multi-character pattern before repeating it. [S1] [S2]

Here are examples to use in either language:

• Optional a: a? — an empty string or a.
• Zero or more a characters: a* — an empty string, a, or aaaa.
• One or more a characters: a+ — a or aaaa, but not an empty string.
• Exactly three a characters: a{3} — aaa.
• Exactly two ha groups: (ha){2} — haha.

These examples illustrate the documented quantifier and grouping rules. [S1] [S2] They describe what the pattern can match; whether other text is allowed before or after a match depends on how the pattern is used.

First identify the exact character or sequence that should repeat. If it has multiple characters, put parentheses around the whole sequence, then append the quantifier. Check what is immediately before the quantifier: that is the item it applies to. [S1] [S2]

For example, if the goal is exactly two ha groups, write (ha){2}. It matches haha. The similar-looking ha{2} is different: its quantifier applies only to a, so it matches haa. [S1] [S2]

You can also use a quantifier after a group to specify a different repetition rule: (ha)* allows zero or more copies of the group, while (ha)+ requires one or more. Choose based on whether the whole group is optional or must appear at least once. [S1] [S2]