JAVA REGEX :: Could you explain this?

Question

My pattern is [a-z][\\*\\+\\-_\\.\\,\\|\\s]?\\b

My Result:

a__
not matched
a_.
pattern matched = a_
a._
pattern matched = a.
a..
pattern matched = a

why my first input is alone not matched??? Thanks in advance.

[ PS: got the same result with [a-z][\\*\\+\\-\\_\\.\\,\\|\\s]?\\b ]

What exactly do you want to match? Can you explain in words? — isnot2bad
– isnot2bad, Commented Nov 18, 2014 at 7:23
You don't need to escape meta-characters in word classes (between [])! — isnot2bad
– isnot2bad, Commented Nov 18, 2014 at 7:28
@isnot2bad is right; no need to escape metacharacters in character classes. Your regex could be rewritten as "[a-z][*+\\-_.,|\\\\s]?\b"; also, not sure about the last term: didn't you mean to match a space character? — fge
– fge, Commented Nov 18, 2014 at 7:32

Amadan · Accepted Answer · 2014-11-18 07:23:17Z

2

Because unlike the period ., the underscore _ is considered to be a word character; so a_ is one word, but a. is a word with interpunction.

So, a__ matches a, then matches _, then fails to match a word boundary (since the next _ is a part of the same word).

a.. matches a, skips the character range, then matches the word boundary between the word a and the interpunction ..

answered Nov 18, 2014 at 7:23

Amadan

200k23 gold badges252 silver badges321 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Prabhu R.D Over a year ago

hi, Is it prints the WordBoundary also. If so how come the result of my second input is a_ If not how come the result of my 3rd input is a.

Amadan Over a year ago

Your second one detects a word boundary between word-character _ and non-word-character .. Same with the third example, but in reverse (the third example has two words separated by interpunction, the second has a single word a_ and interpunction).

fge · Accepted Answer · 2014-11-18 10:03:52Z

2

With the regex rewritten in a "proper way", that is:

"[a-z][*+\\-_.,|\\s]?\\b"

Or, in an "unquoted", canonical way:

[a-z][*+\-_.,|\s]?\b

that your first input does not match is expected; a character class will only ever match one character. After it matches the first underscore, it looks for a word boundary, but cannot find one: for the Java regex engine, _ is a character which can be part of a word. Hence the result.

edited Nov 18, 2014 at 10:03

answered Nov 18, 2014 at 7:34

fge

122k35 gold badges266 silver badges340 bronze badges

2 Comments

nhahtdh Over a year ago

Your rewritten regex messed up the \\s part. The original regex is "[a-z][\\*\\+\\-_\\.\\,\\|\\s]?\\b", in raw form [a-z][\*\+\-_\.\,\|\s]?\b and should be rewritten as [a-z][*+_.,|\s-]?\b, and as string literal "[a-z][*+_.,|\\s-]?\\b"

fge Over a year ago

Sorry, but the post has been edited since then; and it was indeed \\\\s in the original

Collectives™ on Stack Overflow

JAVA REGEX :: Could you explain this?

2 Answers 2

2 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related