How to remove all non printable characters in a string?

7 bit ASCII?

If your Tardis just landed in 1963, and you just want the 7 bit printable ASCII chars, you can rip out everything from 0-31 and 127-255 with this:

$string = preg_replace('/[\x00-\x1F\x7F-\xFF]/', '', $string);

It matches anything in range 0-31, 127-255 and removes it.

8 bit extended ASCII?

You fell into a Hot Tub Time Machine, and you're back in the eighties. If you've got some form of 8 bit ASCII, then you might want to keep the chars in range 128-255. An easy adjustment - just look for 0-31 and 127

$string = preg_replace('/[\x00-\x1F\x7F]/', '', $string);

UTF-8?

Ah, welcome back to the 21st century. If you have a UTF-8 encoded string, then the /u modifier can be used on the regex

$string = preg_replace('/[\x00-\x1F\x7F]/u', '', $string);

This just removes 0-31 and 127. This works in ASCII and UTF-8 because both share the same control set range (as noted by mgutt below). Strictly speaking, this would work without the /u modifier. But it makes life easier if you want to remove other chars...

If you're dealing with Unicode, there are potentially many non-printing elements, but let's consider a simple one: NO-BREAK SPACE (U+00A0)

In a UTF-8 string, this would be encoded as 0xC2A0. You could look for and remove that specific sequence, but with the /u modifier in place, you can simply add \xA0 to the character class:

$string = preg_replace('/[\x00-\x1F\x7F\xA0]/u', '', $string);

Addendum: What about str_replace?

preg_replace is pretty efficient, but if you're doing this operation a lot, you could build an array of chars you want to remove, and use str_replace as noted by mgutt below, e.g.

//build an array we can re-use across several operations$badchar=array(    // control characters    chr(0), chr(1), chr(2), chr(3), chr(4), chr(5), chr(6), chr(7), chr(8), chr(9), chr(10),    chr(11), chr(12), chr(13), chr(14), chr(15), chr(16), chr(17), chr(18), chr(19), chr(20),    chr(21), chr(22), chr(23), chr(24), chr(25), chr(26), chr(27), chr(28), chr(29), chr(30),    chr(31),    // non-printing characters    chr(127));//replace the unwanted chars$str2 = str_replace($badchar, '', $str);

Intuitively, this seems like it would be fast, but it's not always the case, you should definitely benchmark to see if it saves you anything. I did some benchmarks across a variety string lengths with random data, and this pattern emerged using php 7.0.12

     2 chars str_replace     5.3439ms preg_replace     2.9919ms preg_replace is 44.01% faster     4 chars str_replace     6.0701ms preg_replace     1.4119ms preg_replace is 76.74% faster     8 chars str_replace     5.8119ms preg_replace     2.0721ms preg_replace is 64.35% faster    16 chars str_replace     6.0401ms preg_replace     2.1980ms preg_replace is 63.61% faster    32 chars str_replace     6.0320ms preg_replace     2.6770ms preg_replace is 55.62% faster    64 chars str_replace     7.4198ms preg_replace     4.4160ms preg_replace is 40.48% faster   128 chars str_replace    12.7239ms preg_replace     7.5412ms preg_replace is 40.73% faster   256 chars str_replace    19.8820ms preg_replace    17.1330ms preg_replace is 13.83% faster   512 chars str_replace    34.3399ms preg_replace    34.0221ms preg_replace is  0.93% faster  1024 chars str_replace    57.1141ms preg_replace    67.0300ms str_replace  is 14.79% faster  2048 chars str_replace    94.7111ms preg_replace   123.3189ms str_replace  is 23.20% faster  4096 chars str_replace   227.7029ms preg_replace   258.3771ms str_replace  is 11.87% faster  8192 chars str_replace   506.3410ms preg_replace   555.6269ms str_replace  is  8.87% faster 16384 chars str_replace  1116.8811ms preg_replace  1098.0589ms preg_replace is  1.69% faster 32768 chars str_replace  2299.3128ms preg_replace  2222.8632ms preg_replace is  3.32% faster

The timings themselves are for 10000 iterations, but what's more interesting is the relative differences. Up to 512 chars, I was seeing preg_replace alway win. In the 1-8kb range, str_replace had a marginal edge.

I thought it was interesting result, so including it here. The important thing is not to take this result and use it to decide which method to use, but to benchmark against your own data and then decide.

Many of the other answers here do not take into account unicode characters (e.g. öäüßйȝîûηыეமிᚉ⠛ ). In this case you can use the following:

$string = preg_replace('/[\x00-\x08\x0B\x0C\x0E-\x1F\x7F-\x9F]/u', '', $string);

There's a strange class of characters in the range \x80-\x9F (Just above the 7-bit ASCII range of characters) that are technically control characters, but over time have been misused for printable characters. If you don't have any problems with these, then you can use:

$string = preg_replace('/[\x00-\x08\x0B\x0C\x0E-\x1F\x7F]/u', '', $string);

If you wish to also strip line feeds, carriage returns, tabs, non-breaking spaces, and soft-hyphens, you can use:

$string = preg_replace('/[\x00-\x1F\x7F-\xA0\xAD]/u', '', $string);

Note that you must use single quotes for the above examples.

If you wish to strip everything except basic printable ASCII characters (all the example characters above will be stripped) you can use:

$string = preg_replace( '/[^[:print:]]/', '',$string);

For reference see http://www.fileformat.info/info/charset/UTF-8/list.htm

Starting with PHP 5.2, we also have access to filter_var, which I have not seen any mention of so thought I'd throw it out there. To use filter_var to strip non-printable characters < 32 and > 127, you can do:

Filter ASCII characters below 32

$string = filter_var($input, FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_LOW);

Filter ASCII characters above 127

$string = filter_var($input, FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_HIGH);

Strip both:

$string = filter_var($input, FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_LOW|FILTER_FLAG_STRIP_HIGH);

You can also html-encode low characters (newline, tab, etc.) while stripping high:

$string = filter_var($input, FILTER_UNSAFE_RAW, FILTER_FLAG_ENCODE_LOW|FILTER_FLAG_STRIP_HIGH);

There are also options for stripping HTML, sanitizing e-mails and URLs, etc. So, lots of options for sanitization (strip out data) and even validation (return false if not valid rather than silently stripping).

Sanitization: http://php.net/manual/en/filter.filters.sanitize.php

Validation: http://php.net/manual/en/filter.filters.validate.php

However, there is still the problem, that the FILTER_FLAG_STRIP_LOW will strip out newline and carriage returns, which for a textarea are completely valid characters...so some of the Regex answers, I guess, are still necessary at times, e.g. after reviewing this thread, I plan to do this for textareas:

$string = preg_replace( '/[^[:print:]\r\n]/', '',$input);

This seems more readable than a number of the regexes that stripped out by numeric range.