What does [[:print:]] means on the following code?

echo 255 > /sys/devices/platform/[[:print:]]*/hwmon/hwmon[[:print:]]*/pwm1

What is the difference between using [[:print:]]* and only *?

echo 255 > /sys/devices/platform/*/hwmon/hwmon*/pwm1

Is there any known name of this feature, or any place I could read and understand more about?

CodePudding user response：

[[:print:]], in either a glob-style expression or a POSIX-compliant regex, matches any printable character.

The reference for (the simplified, single-character case of) glob expressions is https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_13_01; it references the regular expression portion of the standard at https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_05 as being authoritative for square-bracket expressions, which describes [:print:] as one of the character-class expressions that all locales must provide. The details of this specific class are then provided in https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap07.html#tag_07_03_01.

CodePudding user response：

[[:print:]] is a Posix class recognized by all engines today as matching up to
143,708 or less, Unicode characters as of Unicode 13.
The raw representation of that is this UTF-8/32 regex based on UCD.
This matches up to ALL of them. Some engines match less, doesn't matter though,
it is always this or less, never more as of V13.

(?:[\t-\n-~\\ -¬®-ͷͺ-Ϳ΄-ΊΌΎ-ΡΣ-ԯԱ-Ֆՙ-֊֍-֏֑-ׇא-תׯ-״؆-؛؞-ۜ۞-܍ܐ-݊ݍ-ޱ߀-ߺ߽-࠭࠰-࠾ࡀ-࡛࡞ࡠ-ࡪࢠ-ࢴࢶ-ࣇ࣓-ࣣ࣡-ঃঅ-ঌএ-ঐও-নপ-রলশ-হ়-ৄে-ৈো-ৎৗড়-ঢ়য়-ৣ০-৾ਁ-ਃਅ-ਊਏ-ਐਓ-ਨਪ-ਰਲ-ਲ਼ਵ-ਸ਼ਸ-ਹ਼ਾ-ੂੇ-ੈੋ-੍ੑਖ਼-ੜਫ਼੦-੶ઁ-ઃઅ-ઍએ-ઑઓ-નપ-રલ-ળવ-હ઼-ૅે-ૉો-્ૐૠ-ૣ૦-૱ૹ-૿ଁ-ଃଅ-ଌଏ-ଐଓ-ନପ-ରଲ-ଳଵ-ହ଼-ୄେ-ୈୋ-୍୕-ୗଡ଼-ଢ଼ୟ-ୣ୦-୷ஂ-ஃஅ-ஊஎ-ஐஒ-கங-சஜஞ-டண-தந-பம-ஹா-ூெ-ைொ-்ௐௗ௦-௺ఀ-ఌఎ-ఐఒ-నప-హఽ-ౄె-ైొ-్ౕ-ౖౘ-ౚౠ-ౣ౦-౯౷-ಌಎ-ಐಒ-ನಪ-ಳವ-ಹ಼-ೄೆ-ೈೊ-್ೕ-ೖೞೠ-ೣ೦-೯ೱ-ೲഀ-ഌഎ-ഐഒ-ൄെ-ൈൊ-൏ൔ-ൣ൦-ൿඁ-ඃඅ-ඖක-නඳ-රලව-ෆ්ා-ුූෘ-ෟ෦-෯ෲ-෴ก-ฺ฿-๛ກ-ຂຄຆ-ຊຌ-ຣລວ-ຽເ-ໄໆ່-ໍ໐-໙ໜ-ໟༀ-ཇཉ-ཬཱ-ྗྙ-ྼ྾-࿌࿎-࿚က-ჅჇჍა-ቈቊ-ቍቐ-ቖቘቚ-ቝበ-ኈኊ-ኍነ-ኰኲ-ኵኸ-ኾዀዂ-ዅወ-ዖዘ-ጐጒ-ጕጘ-ፚ፝-፼ᎀ-᎙Ꭰ-Ᏽᏸ-ᏽ᐀-᚜ᚠ-ᛸᜀ-ᜌᜎ-᜔ᜠ-᜶ᝀ-ᝓᝠ-ᝬᝮ-ᝰᝲ-ᝳក-៝០-៩៰-៹᠀-᠍᠐-᠙ᠠ-ᡸᢀ-ᢪᢰ-ᣵᤀ-ᤞᤠ-ᤫᤰ-᤻᥀᥄-ᥭᥰ-ᥴᦀ-ᦫᦰ-ᧉ᧐-᧚᧞-ᨛ᨞-ᩞ᩠-᩿᩼-᪉᪐-᪙᪠-᪭᪰-ᫀᬀ-ᭋ᭐-᭼ᮀ-᯳᯼-᰷᰻-᱉ᱍ-ᲈᲐ-ᲺᲽ-᳇᳐-ᳺᴀ-᷹᷻-ἕἘ-Ἕἠ-ὅὈ-Ὅὐ-ὗὙὛὝὟ-ώᾀ-ᾴᾶ-ῄῆ-ΐῖ-Ί῝-`ῲ-ῴῶ-῾ -\ ‐-\
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