regex - Remove all spaces from lines starting with specific word -

using regex find/replace in notepadd++ how can remove spaces line if line starts 'chapter'?

example text:

chapter b c once upon time. 

what want end with:

chapterabc once upon time. 

incorrect code like:

(?<=chapter)( )(?<=\r\n) 

so 'chapter' needs stay , search should stop @ first line break.

you may use \g based regex match line starts chapter , match consecutive non-whitespace , whitespace chunks linebreak while omitting matched non-whitespace chunks , removing horizontal whitespace:

(?:^chapter|(?!^)\g)\s*\k\h+ 

details:

• (?:^chapter|(?!^)\g) - chapter @ start of line (^chapter) or (|) end of previous successful match ((?!^)\g, \g can match start of line, use retricting negative lookahead.)
• \s* - 0 or more non-whitespace symbols
• \k - match reset operator forcing regex engine omit text matched far (thus, not remove chapter or of non-whitespace chunks)
• \h+ - horizontal whitespace (1 or more occurrences) only