Regex to check only if the group is present - regex

Question

I have String which may have values like below.
854METHYLDOPA
041ALDOMET /00000101/
133IODETO DE SODIO [I 131]
In this i need to get the text starting from index 4 till we find any one these patterns /00000101/ or [I 131]
Expected Output:
METHYLDOPA
ALDOMET
IODETO DE SODIO
I have tried the below RegEx for the same
(?:^.{3})(.*)(?:[[/][A-Z0-9\s]+[]/\s+])
But this RegEx works if the string contains [/ but it doesn't work for the case1 where these patterns doesn't exist.
I have tried adding ? at the end but it works fore case 1 but doesn't work for case 2 and 3.
Could anyone please help me on getting the regx work?

Answer 1

Your logic is difficult to phrase. My interpretation is that you always want to capture from the 4th character onwards. What else gets captured depends on the remainder of the input. Should either /00000101/ or [I 131] occur, then you want to capture up until that point. Otherwise, you want to capture the entire string. Putting this all together yields this regex:
^.{3}(?:(.*)(?=/00000101/|\[I 131\])|(.*))
Demo

Answer 2

You may try this:
^.{3}(.*?)($|(?:\s*\/00000101\/)|(?:\s*\[I\s+131\])).*$
and replace by this to get the exact output you want.
\1
Regex Demo
Explanation:
^ --> start of a the string
.{3} --> followed by 3 characters
(.*?) --> followed by anything where ? means lazy it will fetch until it finds the following and won't go beyond that. It also captures it as
group 1 --> \1
($|(?:\s*\/00000101\/)|(?:\s*\[I\s+131\])) ---------->
$ --> ends with $ which means there is there is not such pattern that
you have mentioned
| or
(?:\s*\/00000101\/) -->another pattern of yours improvised with \s* to cover zero or more blank space.
| or
(?:\s*\[I\s+131\]) --> another pattern of yours with improvised \s+
which means 1 or more spaces. ?: indicates that we will not capture
it.
.*$ --> .* is just to match anything that follows and $
declares the end of string.
so we end up only capturing group 1 and nothing else which ensures to
replace everything by group1 which is your target output.

Answer 3

You could get the values you are looking for in group 1:
^.{3}(.+?)(?=$| ?\[I 131\]| ?\/00000101\/)
Explanation
From the beginning of the string ^
Match the first 3 characters .{3}
Match in a capturing group (where your values will be) any character one or more times non greedy (.+?)
A positive lookahead (?=
To assert what follow is either the end of the string $
or |
an optional space ? followed by [I 131] \[I 131\]
or |
an optional space ? followed by /00000101/ \/00000101\/
If your regex engine supports \K, you could try it like this and the values you are looking for are not in a group but the full match:
^.{3}\K.+?(?=$| ?\[I 131\]| ?\/00000101\/)