Question

I have a regex, for example (ma|(t){1}). It matches ma and t and doesn’t match bla.

I want to negate the regex, thus it must match bla and not ma and t, by adding something to this regex. I know I can write bla, the actual regex is however more complex.

Answer 1

Use negative lookaround: (?!pattern)

Positive lookarounds can be used to assert that a pattern matches. Negative lookarounds is the opposite: it’s used to assert that a pattern DOES NOT match. Some flavor supports assertions; some puts limitations on lookbehind, etc.

Links to regular-expressions.info

• Lookahead and Lookbehind Zero-Width Assertions
• Flavor comparison

See also

• How do I convert CamelCase into human-readable names in Java?
• Regex for all strings not containing a string?
• A regex to match a substring that isn’t followed by a certain other substring.

More examples

These are attempts to come up with regex solutions to toy problems as exercises; they should be educational if you’re trying to learn the various ways you can use lookarounds (nesting them, using them to capture, etc):

• codingBat plusOut using regex
• codingBat repeatEnd using regex
• codingbat wordEnds using regex