Need RegEx Pattern to get text between delimiters at start of text - regex

Question

My source text could be any number of characters between "[" an "]" at the beginning of the line. I will have ONLY one line.
For example:
[1] and some other text here
[10] more text, but maybe some brackets [KEY]
[1000000] a lot more text
I want to match/return the text between the "[" and "]".
EDIT AFTER ANSWER PROVIDED
The first answer, provided by #nickb worked for me with this AppleScript:
Note that I had to convert the RegEx to a quoted string to use in AS. This uses the Satimage AppleScript Additions find text command, which provides the RegEx engine for AppleScript.
set strRegEx to "^\\[(.*?)\\]" -- Original: "^\[(.*?)\]"
set strTextToSearch to "[10] My Note title with [KEY] "
set strCaptureGroup to find text strRegEx in strTextToSearch using {"\\1"} with regexp and string result
log strCaptureGroup
-->10

Answer 1

The most simple regex you could use would be this:
^\[(.*?)\]
You can see it matching your input here.

Answer 2

Alternatively a pure AppleScript solution
set theText to "[1] and some other text here
[10] more text, but maybe some brackets [KEY]
[1000000] a lot more text"
set resultList to {}
set {TID, text item delimiters} to {text item delimiters, "]"}
repeat with aLine in (get paragraphs of theText)
if aLine starts with "[" then set end of resultList to text 2 thru -1 of text item 1 of aLine
end repeat
set text item delimiters to TID
resultList -- {"1", "10", "1000000"}

Answer 3

I think this will fit your criteria:
^\[([^]]*)\].*
With the stuff in brackets in the first matching group returned.

Answer 4

You can try runing the following reg. exp. on each line:
[^\[]\w+[^\]]
I tested it at regex101 and it matches the contents inside the [], excluding the brackets.

Answer 5

/^\[(.*?)\]/
is really the most simple regex for this case, but it matches surrounding brackets too.
The exact value (without brackets) is stored in 1st capture group.
If you don't want to match brackets, you will need this:
/(?<=^\[).*?(?=\])/
… unless you're using JavaScript – unfortunately, JS doesn't support lookbehinds.
In this case you'll need this regex:
/^[^\[\]]+/
(assuming that every input will start with […] component, and will not be empty)

Answer 6

The regex to use depends on how you are going to use it for the input it will parse. Some of the answers here have a trailing .* and some do not. Both are correct, it just depends on what exactly you are trying to match, and crucially how you ask it about a match. For example, in Java, with the regex ^\[(.*?)\], if you feed it the whole string "[1000000] a lot more text" and call matches(), it will return false because the regex pattern does not account for any of the trailing text outside the brackets. However, if you call find() after feeding in the same string, it will match because find() works on each substring as it parses and will return true on the first match it hits, while matches() will only return true if the entire input matches the regex. find() will also find subsequent substring matches to the regex in the string each time find() is called until the parser reaches the end of the input.
Personally, I like to use regex that account for the entire input and use capture groups to isolate the actual text I want to grab from the input. But your mileage may vary.