How to match regex for specific character at start, middle and end

Question

I have a string which is invalid if it has

  _ at start ||
  _ at end ||
  more than one consecutive _ in the middle

but it is valid if more than one non consecutive _ found in a string

examples are

"'test_test_test" - valid 
"test_" - invalid
"_test" - invalid
"test_____test" - invalid

I am trying different pattern to match all this condition into one but doesn't seem to work for e.g.("^[__]+") or ("^[_]+") or ("^[_]$+")

If you could help me with regex and some explanation, I would really appreciate!

The easiest pattern is `^[^_]+(?:_[^_]+)*$`, adapted [from this solution](https://stackoverflow.com/a/4897384/3832970). — Wiktor Stribiżew, Jan 15 '20 at 11:01
`test_` and `_test` are not matched with `^[^_]+(?:_[^_]+)*$` — Wiktor Stribiżew, Jan 15 '20 at 11:05

score 0 · Answer 1 · answered Jan 15 '20 at 11:12

0

You can use below code:

Pattern p = Pattern.compile("^(_\\w*)|(\\w*_{2,}+\\w*)|(\\w*_$)");
Matcher m = p.matcher("te_st_");
System.out.println("Is string invalid: "+ m.matches());

See the result: https://ideone.com/hwMzL1

For more details: Pattern

answered Jan 15 '20 at 11:12

Sumit Singh

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How to match regex for specific character at start, middle and end

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