How to convert a python regexp to java

Question

I need to convert the following python regexp to java regexp:

regexp = re.compile(r"^(?P<prefix>(%s)(%s)?)\s?\b(?P<name>.+)" %
                                ("|".join(array1),
                                 "|".join(array2)), re.IGNORECASE
                                                             | re.UNICODE)

where array1 and 2 are arrays of strings.

What I did is:

String regexp = String.format("^(?<prefix>(%s)(%s)?)\\s?\\b(?<name>.+)", array1, array2);
regexpPattern = Pattern.compile(regexp, Pattern.CASE_INSENSITIVE);

But I get a PatternSyntaxException: "Unknown look-behind group near" in the question mark of (%s)(%s)?

I don't understand very well this question mark.

Any suggestion on how to translate it into Java 1.6?

Hint: the python syntax makes use of named capture groups. If you understand what the python regex does, you should be able to translate it! — devnull
– devnull, Commented Mar 24, 2014 at 9:03

RokL · Accepted Answer · 2014-03-24 09:03:28Z

1

Tons of things will go wrong for you.

The (?< is a positive look-behind expression in java.

(?P<prefix> is a named group in python, there are no named groups in java.

String.format for %s and an array will not produce | joined strings from array like you do in python example.

First you will need to join strings by | from arrays manually. Then when you have two strings, you can do: regexpPattern = Pattern.compile(String.format("^((?:%s)(?:%s)?)\\s?\\b(.+)", string1, string2));

What was prefix group in python is now group 1 in java and name group is group 2.

answered Mar 24, 2014 at 9:03

RokL

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2 Comments

superrache Over a year ago

Thanks a lot, it works perfectly now! OK, named groups don't exist in java. After the pattern compilation, I use the replaceAll function calling the named groups by $1 and $2. And yes, I need to concat my arrays of strings. Thanks for your useful help.

Pshemo Over a year ago

there are no named groups in java. Nonsense. (?<name>regex) works fine in Java as named group, but it was introduced in Java 7. (?<=...) or (?<!...) are look-behind (positive and negative).

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