Regex Query in Java Program

Question

^[0-9]\\d*(\\.\\d+)?$

I can't quite work out what the above regex pattern is looking for. I'm tempted to interpret it as "find anything that is not the numbers 0-9 inclusive, then find zero or more occurrences of a single digit, then find zero or one occurrences of a decimal point followed by at least one digit" but I'm not sure.

Part of my confusion stems from the fact that in the SCJP6 certification book, the not operator is included inside the square brackets, whereas here it's outside. Also, I am just generally inexperience when it comes to regex.

Can someone please help? [This is from a Java program. Is the above in any way Java specific?] Thanks.

jlordo · Accepted Answer · 2013-01-07 11:10:40Z

3

^ start of a string

[0-9] a single digit

\\d* any amount of digits (0-infinity)

(\\.\\d+)? Once, or not at all: a dot followed by at least one digit

$ end of string.

You have a complicated regex that will match any floating point or non floiting point number.

Have a look at the java.util.Pattern class and and the Oracle Java Regex Tutorial.

answered Jan 7, 2013 at 11:10

jlordo

37.9k7 gold badges63 silver badges84 bronze badges

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

Comments

Peter Lawrey · Accepted Answer · 2013-01-07 11:11:59Z

2

It is looking a one or more digits, optionally followed by a . and one or more digits. It is confusing as it is needlessly complicated. It is the same as

^\\d+(\\.\\d+)?$

\d is defined as A digit: [0-9]

answered Jan 7, 2013 at 11:11

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Comments

epoch · Accepted Answer · 2013-01-07 11:10:47Z

1

When the "^" operator is outside of a character class "[]" it denotes the start of input, "$" defines end of input.

So your description is correct, but it should be changed to:

find a single digit from zero to nine...

for more information about regular expressions check out this link

answered Jan 7, 2013 at 11:10

epoch

16.7k5 gold badges50 silver badges72 bronze badges

2 Comments

ConorW Over a year ago

So what is the difference between [0-9] and \\d?

Peter Lawrey Over a year ago

There is no difference between [0-9] and \\d

Collectives™ on Stack Overflow

Regex Query in Java Program

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related