So far, we've seen how we can repeat a pattern multiple times, but we are restricted in what patterns we can repeat. If we want to repeat individual characters, we're covered, but what if we want a repeating sequence of characters? Enclosing any set of patterns in parentheses allows them to be treated as a single pattern when applying repetition operations. Compare these patterns:
'abccc' matches pattern 'abc{3}'
'abccc' does not match pattern '(abc){3}'
'abcabcabc' matches pattern '(abc){3}'
Combined with complex patterns, this grouping feature greatly expands our pattern-matching repertoire. Here's a regular expression that matches simple English sentences:
'Eat.' matches pattern '[A-Z][a-z]*( [a-z]+)*\.$' 'Eat more good food.' matches pattern '[A-Z][a-z]*( [a-z]+)*\.$' 'A good meal.' matches pattern '[A-Z][a-z]*( [a-z]+)*\.$'
The first word starts with a capital, followed by zero or more lowercase letters. Then, we enter a parenthetical that matches a single space followed by a word of one or more lowercase letters. This entire parenthetical is repeated zero or more times, and the pattern is terminated with a period. There cannot be any other characters after the period, as indicated by the $ matching the end of string.
We've seen many of the most basic patterns, but the regular expression language supports many more. I spent my first few years using regular expressions looking up the syntax every time I needed to do something. It is worth bookmarking Python's documentation for the re module and reviewing it frequently. There are very few things that regular expressions cannot match, and they should be the first tool you reach for when parsing strings.