Find and Replace a pattern using Java RegEx in block comments

Question

This is the regex which is used to find block comments and it works absolutely fine

/\\*(?>(?:(?>[^*]+)|\\*(?!/))*)\\*/

I just need to modify it a little bit. Find a semi-colon (;) that "may" exists in the block comments and replace it with a white space.

Currently I am doing this

while (m.find()) {
    if (m.group().contains(";")) {
        replacement = m.group().replaceAll(";", "");
        m.appendReplacement(sb, replacement);
    }
}
m.appendTail(sb);

But I need to replace it with a str.replaceAll kind of statement. In short anything that is more efficient because I get out of memory exception. I fixed a few other regex that used to throw same exception and they are working fine. I hope this regex can also be optimized.

--- Edit ---

These are the string you can test this regex on

/* this* is a ;*comment ; */

/* This ; is* 
another
;*block
comment;
;*/

Thanks

I might write you the Regex if you can post an example phrase and then highlight what you want from it. — Mikhail
– Mikhail, Commented Nov 25, 2011 at 10:07
@Misha Please view the edited question. I have provided a sample String. Thanks — Ali
– Ali, Commented Nov 25, 2011 at 11:07

Alan Moore · Accepted Answer · 2011-11-25 12:09:34Z

3

It'l be much simper to use (?s)/\*.+?\*/ regexp. In your expression you use negative lookahead that "eat" your memory. And your code may be simpler:

while (m.find()) {
    m.appendReplacement(sb, m.group().replace(";","");
}
m.appendTail(sb);

edited Nov 25, 2011 at 12:09

Alan Moore

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answered Nov 25, 2011 at 11:17

temple.t

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5 Comments

Alan Moore Over a year ago

+1 for clearing away the clutter (especially that m.group().contains(";") call), but his regex is optimized for efficiency, so I wouldn't change that. It's not how I would have written it, but it should perform significantly better than /\*.+?\*/.

temple.t Over a year ago

I'm not sure about better efficiency. Lookahead could call problems with memory, especially with large text at entrance. Atomic group can't solve this at all.

Alan Moore Over a year ago

It's only looking ahead for one character, and it only does that after it sees an asterisk. Performance-wise, lookaheads don't cause nearly as many problems as do alternations and quantifiers with overlapping effects. That's what the atomic groups are there for.

Ali Over a year ago

@AlanMoore Can you explain this a little bit "alternations and quantifiers with overlapping effects" .. Examples might be helpful. Thanks

Alan Moore Over a year ago

@Ali: This question demonstrateshow an alternation can bog down a match when two or more alternatives are capable of matching the same characters. As for the quantifiers, see this.

itun · Accepted Answer · 2011-11-25 10:28:32Z

0

There are two variants(try both):

1). Why are you using ?>? I don't know what it means and I don't see a need to use something special here like ?>. Change it to ?:.

2). Your loop is infinite. You need this:

    int index = 0;
    while (m.find(index)) {
        if (m.group().contains(";")) {
            replacement = m.group().replaceAll(";", "");
            m.appendReplacement(sb, replacement);
        }
        index = m.end();
    }

edited Nov 25, 2011 at 10:28

answered Nov 25, 2011 at 10:08

itun

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1 Comment

Alan Moore Over a year ago

(?>...) is an atomic group, and it's necessary for maximum efficiency (possessive quantifiers would work, too). And it's not an infinite loop; the find() method keeps track of the match-start position by itself. find(int) is for special cases.

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