How do I write a regular expression to find the following pattern?

Question

I am trying to write a regular expression to do a find and replace operation. Assume Java regex syntax. Below are examples of what I am trying to find:

• 12341+1
• 12241+1R1
• 100001+1R2

So, I am searching for a string beginning with one or more digits, followed by a "1+1" substring, followed by 0 or more characters. I have the following regex:

^(\d+)(1\\+1).*

This regex will successfully find the examples above, however, my goal is to replace the strings with everything before "1+1". So, 12341+1 would become 1234, and 12241+1R1 would become 1224. If I use the first grouped expression $1 to replace the pattern, I get the wrong result as follows:

• 12341+1 becomes 12341
• 12241+1R1 becomes 12241
• 100001+1R2 becomes 100001

Any ideas?

Answer 1

Your regex looks fine to me - I don't have access to java but in JavaScript the code..

"12341+1".replace(/(\d+)(1\+1)/g, "$1");

Returns 1234 as you'd expect. This works on a string with many 'codes' in too e.g.

"12341+1 54321+1".replace(/(\d+)(1\+1)/g, "$1"); 

gives 1234 5432.

Answer 2

Your existing regex works fine, just that you are missing a \ before \d

String str = "100001+1R2";
str = str.replaceAll("^(\\d+)(1\\+1).*","$1");

Working link

Answer 3

Personally, I wouldn't use a Regex at all (it'd be like using a hammer on a thumbtack), I'd just create a substring from (Pseudocode)

stringName.substring(0, stringName.indexOf("1+1")) 

But it looks like other posters have already mentioned the non-greedy operator.

In most Regex Syntaxes you can add a '?' after a '+' or '*' to indicate that you want it to match as little as possible before moving on in the pattern. (Thus: ^(\d+?)(1+1) matches any number of digits until it finds "1+1" and then, NOT INCLUDING the "1+1" it continues matching, whereas your original would see the 1 and match it as well).

Answer 4

IMHO, the regex is correct.

Perhaps you wrote it wrong in the code. If you want to code the regex ^(\d+)(1\+1).* in a string, you have to write something like String regex = "^(\\d+)(1\\+1).*".

Your output is the result of ^(\d+)(1+1).* replacement, as you miss some backslash in the string (e.g. "^(\\d+)(1\+1).*").