"How to" help with RegExp Find and Replace

[rongin3]

Recently I moved several thousand Whitburn ratings from the "Comment" field to the "Custom 1" field (renamed "Pop Rating") using the ExtractFields.vbs script. Whitburn ratings are usually depicted as "61_248" as an example. After the extraction, the rating appeared in the "Custom 1" field absent the "_" underscore. If it is possible can someone show me entries to use in the RegExp Find and Replace window to replace the "blank" with an "underscore".

Thanks

[rongin3]

I could really use some help if someone has a minute.

Thanks

[onenonymous]

Find What: space (ie type a space)
Into: Custom 1
Replace With: _