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1. Press [Ctrl]+H to open the Find And Replace dialog box.
2. Click inside the Find What and enter two spaces (just two).
3. Click inside the Replace With and enter one space.
4. Click Replace All
I just tested it at my end and it worked fine.
I have the same issue dealing with an older document written with double spacing after a period and I want to replace them all with single space.
In existing topic of mine:
1. Type two spaces into Find What.
2. Type once space into Replace With.
3. Click Replace All and Robohelp says it "Found nothing to replace". There are definitely double spaces in the topic, I checked, so there's something more going on here.
A. Repeated test with a new topic where I typed in "cat. dog" and it finds and replaces it.
B. Went to existing topic with multiple instances of double spaces. Went to one instance, deleted the two spaces and re-typed the two spaces.
Repeated test and one instance was replaced - the one I typed in just before. The other instances of double spaces are left as is.
C. Again in existing topic, went to instance of double space and highlighted the two spaces and copied and pasted them to the clipboard.
Pasted that into the Find What and repeated test. This time all the instances of double spaces were found and replaced in the existing topic.
D. The new topic I created in test A exhibited the same behavior as the existing topic once I saved, closed, and reopened it. The double spaces weren't found.
I don't know exactly what this means, but it seems like Robohelp is transforming the double spaces somewhere behind the scenes. After you close/reopen the topic, the double spaces must not be the same as when you first typed them in.
So it looks like you can either:
1. Copy and paste "transformed" double spaces from the topic into the Find What field.
2. Do the find and replace in the HTML view, which gives a clearer idea of what's going on. My double spaces were "  " i.e. a space followed by a   character.
Enter   in Find What
Enter nothing in Replace With
Then do your find/replace.
One of the issues you might encounter would be where a sentence ends with a space and a new one begins with a space. And in that case, it could be that it's not finding the two spaces together because they are occurring on different lines in the HTML code behind the scenes.
To confirm this, find a spot where you have two spaces that didn't get converted. Select across the two spaces and click the HTML tab and note what is highlighted in the HTML view.