Grouping
A group in a regular expression serves several different possible purposes:
- To denote repetition (of more than a single character)
- To restrict alternation to a part of the regular expression
- To capture values
Repeating groups
Groups may be repeated using any of the quantifiers. The regular expression that tentatively identifies an IP address can be improved using a repeated group. The starting point for this expression is as follows:
[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+
In this expression, the [0-9]+ term followed by a literal . character is repeated three times. Therefore, the expression can become as follows:
([0-9]+\.){3}[0-9]+
The expression itself is not specific, it will match much more than an IP address, but is also now more concise. This example will be explored further later in this chapter.
If * is used as the quantifier for the group, it becomes optional. If faced with a set of version numbers ranging...