TextPad always had the ability to use posix notation, but you have to change the settings in a different dialog box. To use TextPad's default settings for regular expressions, you have to "escape" the opening and closing parentheses:
Replace space after 5-digit zip code, at the beginning of each line
As above, the ^ means start of line
\( is an "escaped parenthesis" and it marks the beginning of the first search expression, i.e., the five digits
[0-9]+ means one or more digits (not just 5-digit zip codes)
\) is another "escaped parenthesis" to mark the end of the first search expression
[ ] is just a space character (you could leave out the brackets, but then no one would be able to see it on this web page :-)
In the replacement expression
\1 is the first search expression, the part between parentheses above (one or more digits)
\t is a tab character
So the search and replace command looks for one or more digits, followed by a space. Then it replaces all that with the same group of digits followed by a tab.
I don't think there is any way simply to find "a space that comes after 5 digits" so you can just replace the space without touching the digits. You have to find the 5 digits (the first string) followed by the space (the second string). Then, although it seems redundant or cumbersome, REPLACE the original string of 5 digits with ITSELF, followed by the tab (the second string).
Everyone who knows this forgets that newbies have no idea about this. That's why I'm spelling it out for you, my friend.
Math Tutor and retired Computer Programmer
New York City