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List:       busybox
Subject:    performance: ${str#????} vs ${str:4}
From:       Alin Mr <almr.oss () outlook ! com>
Date:       2021-07-28 7:08:58
Message-ID: AM8PR08MB57957BD2786B12ED873641ECF7EA9 () AM8PR08MB5795 ! eurprd08 ! prod ! outlook ! com
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Surprisingly, string matching is faster than substring-ing. 

# s is ':' repeated (6 * 2^i) times
s=$(s=::::::; i=12; while [ $i -gt 0 ]; do i=$((i-1)); s="$s$s"; done; echo "$s") \
hyperfine -w2 -S bbox-master/sh  'while [ "$s" ]; do s=${s#????}; done' 'while [ "$s" \
]; do s=${s:4}; done'

Summary
  'while [ "$s" ]; do s=${s#????}; done' ran
    1.16 ± 0.00 times faster than 'while [ "$s" ]; do s=${s:4}; done'

The slowdown doesn't depend much on the skip parameter -- ${s:1} as well as ${s:16} \
are slower than corresponding pattern matches. There are jump points if $s gets much \
longer or much shorter.

I'm curious -- the SUBSTR code seems to check for quotes and CTLESC in vstr, so I \
guess... it can substring in unprocessed (shell) strings? When does this come into \
play -- it's not like one can write ${"a\\bc":2}? \
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