• Finding Lines Containing or Not Containing Certain Words

    If a line can meet any out of series of requirements, simply use alternation in the regular expression.^.*\b(one|two|three)\b.*$ matches a complete line of text that contains any of the words "one", "two" or "three". The first backreference will contain the word the line actually contains. If it contains more than one of the words, then the last (rightmost) word will be captured into the first backreference. This is because the star isgreedy. If we make the first star lazy, like in ^.*?\b(one|two|three)\b.*$, then the backreference will contain the first (leftmost) word.
    If a line must satisfy all of multiple requirements, we need to use lookahead.^(?=.*?\bone\b)(?=.*?\btwo\b)(?=.*?\bthree\b).*$ matches a complete line of text that contains all of thewords "one", "two" and "three". Again, the anchors must match at the start and end of a line and the dot must not match line breaks. Because of the caret, and the fact that lookahead is zero-length, all of the three lookaheads are attempted at the start of the each line. Each lookahead will match any piece of text on a single line (.*?) followed by one of the words. All three must match successfully for the entire regex to match. Note that instead of words like\bword\b, you can put any regular expression, no matter how complex, inside the lookahead. Finally, .*$ causes the regex to actually match the line, after the lookaheads have determined it meets the requirements.
    If your condition is that a line should not contain something, use negative lookahead. ^((?!regexp).)*$ matches a complete line that does not match regexp. Notice that unlike before, when using positive lookahead, I repeated both the negative lookahead and the dot together. For the positive lookahead, we only need to find one location where it can match. But the negative lookahead must be tested at each and every character position in the line. We must test that regexp fails everywhere, not just somewhere.
    Finally, you can combine multiple positive and negative requirements as follows:^(?=.*?\bmust-have\b)(?=.*?\bmandatory\b)((?!avoid|illegal).)*$. When checking multiple positive requirements, the .* at the end of the regular expression full of zero-length assertions made sure that we actually matched something. Since the negative requirement must match the entire line, it is easy to replace the .* with the negative test.
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