Regular expressions are a terse way to do certain simple string processing tasks. For example, to replace all instances of foo in one string with bar, the following can be used:

That could be done with sequence operations, but consider doing this replacement for an arbitrary number of o's, at least two:

The + operator matches one or more occurrences of the previous expression; in this case o. Another useful feature is alternation. Say we want to do this replacement with fooooo or boooo. Then we could use the code

To search a file for all lines that match a given regular expression, you could use code like this:

To test if a string in its entirety matches a regular expression, the following can be used:

Regular expressions can't be used for all parsing tasks. For example, they are not powerful enough to match balancing parentheses.