python regex match number followed by string or nothing

Question

how to match a number followed immediately by either /tcp or /udp or nothing?

the following illustrates 3 scenarios: input --> result

123/tcp_haha --> 123
123 --> 123
123abc/tcp --> no match

i used re.compile(r'(\d+)(?:\/[tcpud]{3})*')but it also matched in case 3.

EDIT: Guess it's really a follow up question: how to match digits either followed by /tcp or /udp or proceeded by tcp/ or /udp or just by itself? so

 1.    something else 123/tcp_haha --> 123
 2.    123  --> 123
 3.    123abc/tcp --> no match
 4.    udp/123 something else --> 123
 5.    tcp/123/tcp --> 123

Kasravnd · Accepted Answer · 2016-11-17 21:22:09Z

4

Character class will match any combinations of its included characters. You need to use a logical OR instead.

r'^\d+(?:/tcp|/udp)?$'

?: is a non-capture group notation and ? will make your non-capture group optional (for non-suffix cases).

If you want to capture the string if something followed the /tcp you can use following regex:

r'^\d+(?:/tcp.*|/udp)?$'

Demo: https://regex101.com/r/oUm0e9/1

edited Nov 17, 2016 at 21:22

answered Nov 17, 2016 at 21:10

Kasravnd

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4 Comments

totoro Over a year ago

thanks for the reply! i just updated the post, realizing that i also want to capture the number even if something follows /tcp. how to achieve that? thanks again

totoro Over a year ago

hi @kas thanks for the reply again. I added a follow up question would be great if you could have a look. if it's too involved, i'll move it to a separate question.

Kasravnd Over a year ago

@dragon_cat Just add the same thing to the leading. r'^(?:tcp/|udp/)?\d+(?:/tcp.*|/udp)?$'

totoro Over a year ago

yes, i have re.compile(r'(?:.*tcp/|.*udp/)?(\d+)(?:/tcp.*|/udp.*)?$'), but this requires both left and right to be either /tcp /udp or none. it will not match say apple123/tcp, which i would like to match.

Matthew · Accepted Answer · 2016-11-17 23:06:06Z

1

How about this for your original problem:

^\d+(?=/tcp|/udp|$)

edited Nov 17, 2016 at 23:06

answered Nov 17, 2016 at 22:15

Matthew

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Comments

Nicolas · Accepted Answer · 2016-11-17 21:18:07Z

0

Use this regex after your edit:

\d+(?=\/tcp|\/udp)

\d+ matches one or more digit
(?=...) is a lookahead, match will stop if next characters are ...
\/tcp|\/udp matches a slash followed by tcp or udp

answered Nov 17, 2016 at 21:18

Nicolas

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1 Comment

totoro Over a year ago

thanks for the reply! it works except fails to match all numbers (case 2)

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