In my classical music collection I use the Year Tag for date of composition; and the Comment Tag for date of recording. I do this with the use of square brackets at the beginning of the Comment Tag, followed by label and catalogue number.
However, I would like now to take the year of recording contained within the square brackets and transfer it (without brackets) to the Recording Date Tag which I have created, but leave behind the label and catalogue number. e.g.
From this - Comment Tag:  Sony Classics 123456;
To this - Comment Tag: Sony Classics 123456; Recording Date Tag: 2019
I realise copying information within brackets from one field to another field seems a common question in this forum, and several examples are given how to achieve this. However, I have had only limited success in adapting the recommendations to my situation using both the Guess Value, and Format Value actions.
e.g. When adapting Ohrenkino's suggestion of using Format Value from Feb 2014. with the following expression: $regexp(%COMMENT%,'^. [(.+)]. $','$1') I get  Sony 123456 in Recording Date tag instead of just the year.
Similarly, when using Guessing Value I am unable to limit the copying to just the year within the square brackets. I am at a loss to know where to go from here, and I would appreciate very much any assistance given in solving this.
Thanks in anticipation.