![]() matches should be longest match leftmost in text.It's also complicated by the fact that many ERE engines nowadays will honor backreferences, which can't efficiently be matched with DFA/"text-directed" techniques. However, this is nowadays complicated by the fact that POSIX specifies the longest possible leftmost stretch of source text be matched: so a POSIX-conformant grep (which accepted pattern alternation as an extension) would have to match "NFA, no I mean DFA", too, just as egrep would. For any regex engine which can handle alternative patterns, printf "NFA, no I mean DFA" | regex_match "/NFA|NFA, no I mean DFA/" would match the whole source text if the engine were a DFA but would match only "NFA", using the left pattern, if the engine were a (naive) NFA. One difference between naive implementations of these is that NFAs are more "eager." Both types of engines will return the leftmost of several possible matches in the source text but the greater eagerness of a NFA would show up in which of several alternative patterns it used to do the matching. There are theoretical translations between these, but in practice devices like backreferences and non-greedy quantification can only be efficiently implemented with NFAs-so although DFAs are faster, they are also more limited. EREs were often implemented using more efficient DFAs or "text-directed" regex engines. Note: Historically, BRE-using regex engines are implemented using NFAs or "pattern-directed" regex engines. Additionally, awk EREs obeys somewhat different rules than other EREs. In practice most implementations of these regex languages go beyond the specification, for example including Gnu extensions like \w, or including features that were only historically available (and are only specified for) the other regex language: thus most implementations of BREs will also honor \+ and \|, and many implementations of EREs will also honor backreferences like \1. EREs are usually also available as an option to sed sometimes this is expressed using sed -r, other times sed -E. The second by tools like egrep ( grep -E), awk, lex, and emacs. The first is historically implemented by utilities like grep, sed, and ed. POSIX defines two classes of regex languages: basic regular expressions (BREs) and extended regular expressions (EREs). If you don't have any length-2 filenames like. Hence, to match all of the four rightmost columns, but neither of the two leftmost ones, you need to combine three glob patterns: *. ![]() How do I construct a shell glob-pattern that matches all files except "." and "." ? (from Unix FAQ 2.11) Todo: What contexts automatically suppress glob-expansion? If set -f / set -o noglob, glob-expansion is disabled.Given the pattern /foo/bar/x*/bam, search permission is needed for directories / and foo, search and read permissions are needed for directory bar, and search permission is needed for each x* directory./ can only be matched literally, and has higher parsing precedence than, so ad only matches file c]d in directory a[b.In BusyBox ash, it's not matched by other shell implementations may differ. A leading period can only be matched literally, not by or ? or * or or ].If the pattern contains an invalid bracket expression or does not match any existing filenames or pathnames, the pattern string will be interpreted literally.can also be expressed in some shells (including BusyBox ash) as but the format is more portable. These are used in shell expansion and pattern-matching in case expressions. Some of the behavior of Alpine's utilities reported here may have changed with the switch to musl. Warning: This summary was drawn up back when we were using uClibc.
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