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To match any character except a list of excluded characters, put the excluded charaters between [^ and ] . The caret ^ must immediately follow the [ or else it stands for just itself. The character '. ' (period) is a metacharacter (it sometimes has a special meaning).
Prepare a Text question type. Go to the question's Settings. Go to Validation Criteria and choose the Manually enter your validation logic in XLSForm code option. In the Validation Code box, enter your regex formula between the quotation marks (' ') of the regex(., ' ') format.
Example: The regex "aa\n" tries to match two consecutive "a"s at the end of a line, inclusive the newline character itself. Example: "a\+" matches "a+" and not a series of one or "a"s. ^ the caret is the anchor for the start of the string, or the negation symbol.
You could use a look-ahead assertion:
(?!999)\d{3}
This example matches three digits other than 999.
But if you happen not to have a regular expression implementation with this feature (see Comparison of Regular Expression Flavors), you probably have to build a regular expression with the basic features on your own.
A compatible regular expression with basic syntax only would be:
[0-8]\d\d|\d[0-8]\d|\d\d[0-8]
This does also match any three digits sequence that is not 999.
If you want to match a word A in a string and not to match a word B. For example: If you have a text:
1. I have a two pets - dog and a cat
2. I have a pet - dog
If you want to search for lines of text that HAVE a dog for a pet and DOESN'T have cat you can use this regular expression:
^(?=.*?\bdog\b)((?!cat).)*$
It will find only second line:
2. I have a pet - dog
Match against the pattern and use the host language to invert the boolean result of the match. This will be much more legible and maintainable.
notnot, resurrecting this ancient question because it had a simple solution that wasn't mentioned. (Found your question while doing some research for a regex bounty quest.)
I'm faced with a situation where I have to match an (A and ~B) pattern.
The basic regex for this is frighteningly simple: B|(A)
You just ignore the overall matches and examine the Group 1 captures, which will contain A.
An example (with all the disclaimers about parsing html in regex): A is digits, B is digits within <a tag
The regex: <a.*?<\/a>|(\d+)
Demo (look at Group 1 in the lower right pane)
Reference
How to match pattern except in situations s1, s2, s3
How to match a pattern unless...
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