Python: re module beginner

Question

I am learning the re module and have a problem understanding re.search and re.match. The documentation gives a good example so I decided to try these out:

>>> import re
>>> pattern = re.compile('\d*')
>>> string = "123"
>>> pattern.match(string)
<_sre.SRE_Match object; span=(0, 3), match='123'>
>>> pattern.search(string)
<_sre.SRE_Match object; span=(0, 3), match='123'>

No surprises here, so I change the string a little:

>>> string = "hello 123"
>>> pattern.match(string)
<_sre.SRE_Match object; span=(0, 0), match=''>
>>> pattern.search(string)
<_sre.SRE_Match object; span=(0, 0), match=''>

Why is nothing found with the last call of search? I would expect it return something like this (a non-empty match object):

<_sre.SRE_Match object; span=(6, 9), match='123'>

What happens during that last call to search? Python 3.4.3

Adam Smith · Accepted Answer · 2015-03-18 17:20:29Z

3

It's your quantifier! The * quantifier matches ZERO or more of the previous atom (in this case a digit \d). The first match returned is zero quantity at position zero.

I can demonstrate:

>>> import re
>>> pattern = re.compile(r"\d*")
>>> string = "hello 123"
>>> pattern.match(string)
<_sre.SRE_Match object; span=(0, 0), match=''>
>>> pattern.search(string)
<_sre.SRE_Match object; span=(0, 0), match=''>
>>> pattern = re.compile(r"\d+") # + is one or more, not zero or more
>>> pattern.match(string) # no match at start of string
>>> pattern.search(string)
<_sre.SRE_Match object; span=(6, 9), match='123'>

answered Mar 18, 2015 at 17:20

Adam Smith

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2 Comments

alisianoi Over a year ago

Oh, I see. However, why does the first search return a successful match and not an empty one then?

Adam Smith Over a year ago

Because the first character it sees IS a digit, so it continues to the next one and the next one. When the first character it sees ISN'T a digit, the regex goes "Oh it's okay, the quantifier says I can have nothing here, too. I guess I have nothing!"

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