java - String.replaceAll to replace all characters not in pattern

Question

I have a Java regex:

^[a-zA-Z_][a-zA-Z0-9_]{1,126}$

It means:

Begin with an alphabetic character or underscore character.
Subsequent characters may include letters, digits or underscores.
Be between 1 and 127 characters in length.

Now, I want to replace a string having characters not in that regex with a underscore.

Example:

final String label = "23_fgh99@#";
System.out.println(label.replaceAll("^[^a-zA-Z_][^a-zA-Z0-9_]{1,126}$", "_"));

But the result is still 23_fgh99@#.

How can I "convert" it to _3_fgh99__?

Because "23_fgh99@#" begins with 2, not [a-zA-Z_], no matches. — Maroun
– Maroun, Commented May 21, 2015 at 10:17
Try label.replaceAll("^[^a-zA-Z_]|(?<!^)[^a-zA-Z0-9_]", "_"). It outputs _3_fgh99__, though. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented May 21, 2015 at 10:17

Wiktor Stribiżew · Accepted Answer · 2015-05-21 10:22:39Z

4

Use this code:

final String label = "23_fgh99@#";
System.out.println(label.replaceAll("^[^a-zA-Z_]|(?<!^)[^a-zA-Z0-9_]", "_"));

It outputs _3_fgh99__.

To remove what is "not in the original pattern", you need to negate the first character class and only check a character at the beginning (^[^a-zA-Z_]), and then check other characters not at the beginning with the negated second character class ((?<!^)[^a-zA-Z0-9_]). Then, we just use an alternation symbol | to apply both patterns in 1 replacement operation.

edited May 21, 2015 at 10:22

answered May 21, 2015 at 10:20

Wiktor Stribiżew

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3 Comments

Maroun Over a year ago

Please try to explain your answer for future users.

Wiktor Stribiżew Over a year ago

@MarounMaroun: Sure, I explain my answers in most cases, and this case seems quite peculiar.

Wiktor Stribiżew Over a year ago

@java_guy: Please consider accepting the answer if it worked for you.

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