Regex Quantifier for exactly one

Question

I have few patterns like:

orange.*
*.key.*
jug.*.key

Here, by * - I mean exactly one word.

So, the pattern *.key.* should match the string bar.key.door (one word on each side) but not fruit.bar.key.door (two words before key ).

How do i write a regex pattern for this in Python?

Answer 1

One way you could match that pattern is to use anchors to assert the start ^ and the end $ of the line and match match the unknown part by using ^[^.]+ (a negated character class ) or use \\w+ to match one or more word characters.

For example:

^[^.]+\\.key\\.[^.]+$

import re
p = re.compile('^[^.]+\.key\.[^.]+$')
m = p.match('bar.key.door')
print (m.group())

Demo

Some options for your other patterns could be: ^orange\\.\\w+$ and ^jug\\.[^.]+\\.key$

Answer 2

There are various ways to implement it:

Option 1: ^[\w]+\.key\.[\w]+$
Option 2: ^[[:word:]]+\.key\.[[:word:]]+$
Option 3: ^[^.]+\.key\.[^.]+$
Option 4: ^[A-Za-z]+\.key\.[A-Za-z]+$

Hope it helps.

Answer 3

. matches any character except a newline. to match the literal . it needs to be escaped, like \\. .

* - means exactly 1 word

* is another regex character meaning match 0 or more repetitions of the previous character group. \\w+ would match a word. a single word would imply the string terminates (before or after), so it can be checked for using ^ or $ .

check out the official re library documentation for more info. It has worked examples and is a very good tutorial for learning regex.

so, the 3 regex patterns would be:

p1 = r'orange\.\w+$'
p2 = r'^\w+\.key\.\w+$'
p3 = r'^jug\.\w+\.key$'