Introduction

We've already used groups in several examples throughout Chapter 2 Regular Expressions with Python. Grouping is accomplished through two metacharacters, the parentheses (). The simplest example of the use of parentheses would be building a subexpression. For example, imagine you have a list of products, the ID for each product being made up of two or three sequences of one digit followed by a dash and followed by one alphanumeric character, 1-a2-b:

>>>re.match(r"(\d-\w){2,3}", ur"1-a2-b")
<_sre.SRE_Match at 0x10f690738>

As you can see in the preceding example, the parentheses indicate to the regex engine that the pattern inside them has to be treated like a unit.

Let's see another example; in this case, we need to match whenever there is one or more ab followed by c:

>>>re.search(r"(ab)+c", ur"ababc")
<_sre.SRE_Match at 0x10f690a08>
>>>re.search(r"(ab)+c", ur"abbc")
None

So, you could use parentheses whenever you want to group meaningful subpatterns...