Question

I knew that [] denotes a set of allowable characters –

>>> p = r'^[ab]$'
>>> 
>>> re.search(p, '')
>>> re.search(p, 'a')
<_sre.SRE_Match object at 0x1004823d8>
>>> re.search(p, 'b')
<_sre.SRE_Match object at 0x100482370>
>>> re.search(p, 'ab')
>>> re.search(p, 'ba')

But … today I came across an expression with vertical bars within parenthesis to define mutually exclusive patterns –

>>> q = r'^(a|b)$'
>>> 
>>> re.search(q, '')
>>> re.search(q, 'a')
<_sre.SRE_Match object at 0x100498dc8>
>>> re.search(q, 'b')
<_sre.SRE_Match object at 0x100498e40>
>>> re.search(q, 'ab')
>>> re.search(q, 'ba')

This seems to mimic the same functionality as above, or am I missing something?

PS: In Python parenthesis themselves are used to define logical groups of matched text. If I use the second technique, then how do I use parenthesis for both jobs?

Answer 1

In this case it is the same.

However, the alternation is not just limited to a single character. For instance,

^(hello|world)$

will match “hello” or “world” (and only these two inputs) while

^[helloworld]$

would just match a single character (“h” or “w” or “d” or whatnot).

Happy coding.