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ksh - Get last character from string - Bad Substitution error

 
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Old 05-03-2014
Quote:
Originally Posted by bakunin
Yes - and no. "regular expressions" is (in a very theoretical sense) any type-3 language in the Chomsky hierarchy: a device where some characters and some metacharacters describe a text pattern. This is the case for shell regexps (aka "file globs") as well as for "Unix Basic Regular Expressions" (what awk, grep and sed use) or "Extended Regular Expressions" (i.e. perl and some GNU variants of grep, sed, ...). These are all different flavours of Regexps (and i should have mentioned that too, probably), but still Regexps nevertheless.
If we are going to be precise with regard to formal language theory, then you are mistaken. Neither POSIX Basic Regular Expressions, nor the "extended" dialects implemented in perl, python, php, java, et al, are regular expressions. Any grammar that supports backreferences cannot be implemented with a [non-]deterministic finite automaton (a defining characteristic of a regular language). sh pattern matching and POSIX Extended Regular Expressions, however, are formally regular languages.

As you noted, I was simply using conventional, informal nomenclature. The ksh/bash man pages make a concerted effort to not use the term 'regular expression' when discussing pattern matching notation.

Minor nit: AWK uses POSIX Extended Regular Expressions, not Basic.

Regards,
Alister
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