Regex to find space at or before the n-th character of a string

Question

I need to find the space character at or before the nth character of a string.

Example, assuming that n is 20: in the string

 Find one space in the right place
^         ^       ^ ^
0         10      M 20

the h is in the 20th position, int this case I need to find the first space before the 20th position, the one before the word "the" in 18th position, just over the M.

I can find the nth character using positive lookbehinds like

(?<=.{80}).

but then I need to go back until I find the first space character and I don't know how.

I'm using notepad++ which doesn't support variable length lookbehinds, if possible I'd like an answer allowing me to use the regex also in notepad++. Thanks.

Answer 1

You can use this regex (inside but excluding quotes):

"^.{1,19} "

Note the space at the end.

I have tested this with Notepad++ 6.1.5 (the latest version at time of posting). According to the feature list, the regex is PCRE compatible (look-behind works, but fixed width only). The regex above doesn't work on an older version I current have (Notepad++ 5.9).

Since you need to replace the space with something else, you may want to use capturing group to preserve the text and replace only the space (inside but excluding quotes):

"^(.{1,19}) " (Find)
"\1replace_space" (Replace)

Note: The regex will search for the space before or at the 20th character. If you want the space to be strictly before 20th character, change 19 to 18.