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copied!<p>To match all allowed fields, the following rather fearful looking regex works:</p> <pre> @((?:cat|mouse|dog|puppy)\b|\((?:(?:cat|mouse|dog|puppy)(?:, *|(?=\))))+\)) </pre> <p>It returns these matches, in order: <code>@cat</code>, <code>@(cat)</code>, <code>@(cat, dog)</code>, <code>@cat</code>, <code>@dog</code>, <code>@(cat, dog)</code>, <code>@mouse</code>.</p> <p>The regex breaks down as follows:</p> <pre> @ # the literal character "@" ( # match group 1 (?:cat|mouse|dog|puppy) # one of your valid search terms (not captured) \b # a word boundary | # or... \( # a literal opening paren (?: # non-capturing group (?:cat|mouse|dog|puppy) # one of your valid search terms (not captured) (?: # non-capturing group , * # a comma "," plus any number of spaces | # or... (?=\)) # a position followed by a closing paren ) # end non-capture group )+ # end non-capture group, repeat \) # a literal closing paren ) # end match group one. </pre> <p>Now to identify any <em>invalid</em> search, you would wrap all that in a negative look-ahead:</p> <pre> @(?!(?:cat|mouse|dog|puppy)\b|\((?:(?:cat|mouse|dog|puppy)(?:, *|(?=\))))+\)) --^^ </pre> <p>This would identify any <code>@</code> character after which an invalid search term (or term combination) was attempted. Modifying it so that it also <em>matches</em> the invalid attempt instead of just pointing at it is not that hard anymore.</p> <p>You would have to prepare <code>(?:cat|mouse|dog|puppy)</code> from your field dynamically and plug it into the static rest of the regex. Should not be too hard to do either.</p>

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