It is signified by a vertical line character | in a regular expression.
Here it is in usage:
A similar thing is square brackets, allowing to choose among multiple characters. For example, pr[ae]y corresponds to pray or prey. But, there is a difference between square brackets and alternation. While square brackets allow only characters or character sets, with alternation you can use any expressions. For instance, a regular expression C|D|E means one of the following expressions: C, D or E.
For applying alternation to a given part of the pattern, it should be enclosed in parentheses, like here:
Regexp for Time: example
Imagine you need to build a regular expression to search time in the hh:mm form ( for example, 11:00), but using a simple\d\d:\d\d can be confusing. It may accept 26:99 as the time (where 99 matches the pattern, and the time is not valid).
To make a better pattern, you should use a more careful matching. The first thing that you should pay attention to is the hours. In case the initial digit is either 0 or 1, the following digit should be \d. Differently, if the initial digit is 2, then the next one should be [0-3]. Note that any other first digit is not allowed.
Writing both of the variants in a regexp with alternation will look like this: \d|2[0-3].
The minutes should be from 00 to 59. In the language of regexp it will be [0-5]\d. After connecting seconds and minutes, the pattern will look as follows: \d|2[0-3]:[0-5]\d.
There is the result but with a problem. The alternation is between \d and 2[0-3]:[0-5]\d . In other words, minutes are inserted to the second alternation variant. It looks like this:
\d | 2[0-3]:[0-5]\d
So, the pattern searches for 2[0-3]:[0-5]\d or \d, which is not correct. The alternation must always be inserted in the hours part of the regexp and allow 2[0-3] or\d.
The final and correct variant will look like this:
So, in this example, “hours” is enclosed into parentheses.