Java Regex Subexpressions

Question

I'm trying to create a regex pattern to match a specific string and return true if the string matches the pattern and false if it doesn't. Here are the conditions:

Must start with [ and end with ]
Each item inside the brackets have to be separated by commas
Each item separated by commas have to follow this regex pattern: ^[A-Za-z][A-Za-z0-9_]*$

How can I make one regex that checks for all these conditions?

No whitespace allowed anywhere?

markspace
– markspace

2016-07-04 18:21:29 +00:00
Commented Jul 4, 2016 at 18:21 — markspace
– markspace, Commented Jul 4, 2016 at 18:21

Zbynek Vyskovsky - kvr000 · Accepted Answer · 2016-07-04 18:30:19Z

3

Enclose in the group which could repeat:

\[[A-Za-z][A-Za-z0-9_]*(?:,[A-Za-z][A-Za-z0-9_])*\]

This is as it should appear in the final string. Escape specials according to specific language.

edited Jul 4, 2016 at 18:30

answered Jul 4, 2016 at 18:23

Zbynek Vyskovsky - kvr000

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1 Comment

Andreas Over a year ago

It is better for regex performance to make the second half repeating (reduces backtracking), and there's no need to capture anything, so: \[[A-Za-z][A-Za-z0-9_]*(?:,[A-Za-z][A-Za-z0-9_]*)*\]

Wiktor Stribiżew · Accepted Answer · 2016-07-04 18:39:15Z

2

In Java, \w without the Pattern.UNICODE_CHARACTER_CLASS flag actually matches the same as [a-zA-Z0-9_]. So, I'd use

String pat = "\\[[a-zA-Z]\\w*(?:,[a-zA-Z]\\w*)*]";

See the IDEONE demo. Use with String#matches, or you will have to add ^ (or \\A) at the beginning and $ (or \\z) at the end.

String pat = "\\[[a-zA-Z]\\w*(?:,[a-zA-Z]\\w*)*]";
System.out.println("[c1,T4,yu5]".matches(pat)); // TRUE

Pattern explanation:

\\[ - a literal [
[a-zA-Z] - an English letter (same as \\p{Alpha})
\\w* - zero or more characters from [a-zA-Z0-9_] set
(?: - start of the non-capturing group matching...
- , - a comma
- [a-zA-Z]\\w* - see above
)* - ... zero or more times
] - a literal ] (does not require escaping outside of the character class to be treated literally).

answered Jul 4, 2016 at 18:39

Wiktor Stribiżew

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2 Comments

Mark Shark Over a year ago

So this is essentially the same regex as Zbynek's except you replaced [a-zA-Z0-9_] with \\w?

Mark Shark Over a year ago

If it is, then both work for me. Thank you for the answer.

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