Python/Regex - match char between two chars, with anything before or after the matching char

Question

I'm trying to match a char within a subset of chars, where either side of the matching char could be anything.

heres an example:

{{ SITE_AGGREGATE_SUBNET }}.3 remote-as {{ BGP-AS }}

against the above, I want to match anything between {{ and }} that has a dash "-" in it.

my regex pattern thus far is:

(?<={{)(.*?-.*?)(?=}})

but this is creating a match for the whole test string returning:

SITE_AGGREGATE_SUBNET }}.3 remote-as {{ BGP-AS

Is anyone able to see what I'm missing? I understand why my regex doesn't work as expected but not how to fix it.

Thanks

anubhava · Answer 1 · 2018-06-25T14:26:37.613

3

You may use this regex with a negative lookahead and a capture group:

({{(?:(?!{{|}})[^-])*)-(.*?}})

RegEx Demo

RegEx Details:

(: Start capture group
- {{: Match {{
- (?:: Start non-capture group
  - (?{{|!}}): Negative lookahead to assert that we don't have {{ and }} at next position
  - [^-]: Match any character except hyphen
- )*: End non-capture group. * matches 0+ instances of this group
): End capture group
-: Match literal hyphen
(.*?}}): Match remaining string up to }} and then match }} and capture this in 2nd capture group

edited Jun 25 '18 at 14:26

answered Jun 25 '18 at 14:02

anubhava

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This will match the text up to the dash, but the OP asks for the whole string inside the double-brackets, as long as said string contains a dash – GPhilo Jun 25 '18 at 14:08
@anubhava.. can you tell me a good ref to start off with regex? – Letsintegreat Jun 25 '18 at 14:11
@DudeCoder: Start with https://www.regular-expressions.info/ a great site to learn regex – anubhava Jun 25 '18 at 14:12
@anubhava.. thanks for that.. but is it that site where you started learning regex? – Letsintegreat Jun 25 '18 at 14:13
I started learning regex in pre-internet days by learning from Automata books :) – anubhava Jun 25 '18 at 14:14
1

Cool, thats amazing – Letsintegreat Jun 25 '18 at 14:15
This solution may match [`{{ {{ BGP-AS }}`](https://regex101.com/r/8VRsFD/5). See [my answer](https://stackoverflow.com/a/51025516/3832970) with the right solution. – Wiktor Stribiżew Jun 25 '18 at 14:23
For nested case of `{{` and `}}` this regex can be easily changed to `({{(?:(?!{{|}})[^-])*)-(.*?}})` – anubhava Jun 25 '18 at 14:25
Yes, it is my solution. Though I am using capturing group in a way for the `re.findall` to return only the contents inside the braces. And since I use `\s*`, that value is stripped off leading/trailing whitespace chars. – Wiktor Stribiżew Jun 25 '18 at 14:28

Wiktor Stribiżew · Accepted Answer · 2018-06-25T14:16:19.443

Use

import re
s = '{{ SITE_AGGREGATE_SUBNET }}.3 remote-as {{ BGP-AS }}'
print([x.strip() for x in re.findall(r'{{(.*?)}}', s) if '-' in x])
// -> ['BGP-AS']

See the Python demo

Details

Extract all matches between {{...}} with a mere {{(.*?)}} regex (note that re.findall will only return the captured substing, the value matched with (.*?))
Only keep the matches with - in them using a condition inside list comprehension (if '-' in x)
Remove trailing/leading whitespace with .strip()

A single regex approach (note it might turn out less efficient):

re.findall(r'{{\s*((?:(?!{{|}})[^-])*-.*?)\s*}}', s)

See the Python demo

Details

{{ - {{
\s* - 0+ whitespaces
((?:(?!{{|}})[^-])*-.*?) - Capturing group 1 (what will be returned by re.findall):
- (?:(?!{{|}})[^-])* - a tempered greedy token matching any non-hyphen char, 0+ times, that does not start a {{ and }} substrings
- - - a hyphen
- .*? - any 0+ chars (other than an LF), as few as possible
\s* - 0+ whitespaces
}} - }}.

See the regex demo

Olivier Melançon · Answer 3 · 2018-06-25T14:16:45.657

You can use this pattern: {{(.*?)}}.

.*? matches any stream of character non-greedily.
(...) creates a capturing group so re.findall yields the inside of the brackets.

To check if the match contains a '-', it might be simpler to then use in.

Code

import re

def tokenize(s):
    return [w.strip() for w in re.findall('{{(.*?)}}', s) if '-' in w]

print(tokenize('{{ SITE_AGGREGATE_SUBNET }}.3 remote-as {{ BGP-AS }}'))

Output

['BGP-AS']

Python/Regex - match char between two chars, with anything before or after the matching char

3 Answers3

Code

Output

Linked