Regex to find all possible occurrences of text starting and ending with ~

Question

I would like to find all possible occurrences of text enclosed between two ~s.

For example: For the text ~*_abc~xyz~ ~123~, I want the following expressions as matching patterns:

1. ~*_abc~
2. ~xyz~
3. ~123~

Note it can be an alphabet or a digit.

I tried with the regex ~[\w]+?~ but it is not giving me ~xyz~. I want ~ to be reconsidered. But I don't want just ~~ as a possible match.

Answer 1

Use capturing inside a positive lookahead with the following regex:

Sometimes, you need several matches within the same word. For instance, suppose that from a string such as ABCD you want to extract ABCD, BCD, CD and D. You can do it with this single regex:

(?=(\w+))

At the first position in the string (before the A), the engine starts the first match attempt. The lookahead asserts that what immediately follows the current position is one or more word characters, and captures these characters to Group 1. The lookahead succeeds, and so does the match attempt. Since the pattern didn't match any actual characters (the lookahead only looks), the engine returns a zero-width match (the empty string). It also returns what was captured by Group 1: ABCD

The engine then moves to the next position in the string and starts the next match attempt. Again, the lookahead asserts that what immediately follows that position is word characters, and captures these characters to Group 1. The match succeeds, and Group 1 contains BCD.

The engine moves to the next position in the string, and the process repeats itself for CD then D.

So, use

(?=(~[^\s~]+~))

See the regex demo

The pattern (?=(~[^\s~]+~)) checks each position inside a string and searches for ~ followed with 1+ characters other than whitespace and ~ and then followed with another ~. Since the index is moved only after a position is checked, and not when the value is captured, overlapping substrings get extracted.

Java demo:

String text = " ~*_abc~xyz~ ~123~";
Pattern p = Pattern.compile("(?=(~[^\\s~]+~))");
Matcher m = p.matcher(text);
List<String> res = new ArrayList<>();
while(m.find()) {
    res.add(m.group(1));
}
System.out.println(res); // => [~*_abc~, ~xyz~, ~123~]

Just in case someone needs a Python demo:

import re
p = re.compile(r'(?=(~[^\s~]+~))')
test_str = " ~*_abc~xyz~ ~123~"
print(p.findall(test_str))
# => ['~*_abc~', '~xyz~', '~123~']