DecimalFormat

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new DecimalFormat(",###").format(new BigDecimal(1234567890).longValue())

String.format

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String.format("%,d",new BigDecimal(1234567890).longValue())

正则表达式

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String.valueOf(new BigDecimal(1234567890).longValue()).replaceAll("(\\d)(?=(\\d{3})+\$)", "\$1,")
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#!/usr/bin/perl

use strict;
use warnings;
my $num = 1234556789;
#纯数据模式
$num =~ s/(?<=\d)(?=(\d{3})+$)/,/g;
print $num,"\n";
#带有小数点模式
$num = 123456.1234;
$num =~ s/(?<!\.\d)(?<=\d)(?=(\d{3})+\b)/,/g;
print $num,"\n";
#大于5位之后。。。
#\K 模式，可变长否定向后环视;\K容许在它之前的模式只匹配而不被替换。替换操作只对\K之后的字符串部分有效。
$num = 123456.12345;
$num =~ s/(?<!\.)(?:\b|\G)\d+?\K(?=(\d{3})+\b)/,/g;
print $num,"\n";
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OUTPUT:
1,234,556,789
123,456.1234
123,456.12345