Python Programming

We previously accepted "b" (with no "a" after the "b"), in addition to ba, baa, baaa, etc.

What if we want at least one "a" after the "b"? Because a "b" without an "a" is not really a sound a sheep would make!

>>> pattern = "baa*"
>>> re.match(pattern, "b") # None
>>> re.match(pattern, "ba")
<re.Match object; span=(0, 2), match='ba'

You can also use a + (Kleene Plus), which represents “one or more times”. This is a shorthand notation for the above! So a+ is the same as aa*.

>>> pattern = "ba+"
>>> re.match(pattern, "b") # None
>>> re.match(pattern, "ba")
<re.Match object; span=(0, 2), match='ba'>
>>> re.match(pattern, "baaaaa")
<re.Match object; span=(0, 6), match='baaaaa'>

Quick task

Write a regular expression that matches one or more x, y or z.

Example valid strings: x, y, z, xx, xy, zxxyx, yxyz, xyzxyz

Example invalid strings: <blank>, aaa, ax

👀 Peek at sample solutions

>>> pattern = "[xyz]+" 
>>> re.match(pattern, "y")
<re.Match object; span=(0, 1), match='y'>
>>> re.match(pattern, "zyxy")
<re.Match object; span=(0, 4), match='zyxy'>
>>> re.match(pattern, "") # None
>>> re.match(pattern, "ax") # None
>>> re.match(pattern, "baaaaa") # None