06-16-2011
matched characters - regular expression
Hi,
I read the book of <<unix shell programming>>. The regular expression ^\(.\)\1 matches the first character on the line and stores it in register 1. Then the expression matches whatever is stored in the register 1, as specified by the \1. The net effect of this regular expression is to match the first two characters on a line if they are both the same character.
I don't fully understand this regular expression, especially "Then the expression matches whatever is stored in the register 1, as specified by the \1." Can someone explain it in detail and more clearly.
Thanks!
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LEARN ABOUT ULTRIX
re_comp
regex(3) Library Functions Manual regex(3)
Name
re_comp, re_exec - regular expression handler
Syntax
char *re_comp(s)
char *s;
re_exec(s)
char *s;
Description
The subroutine compiles a string into an internal form suitable for pattern matching. The subroutine checks the argument string against
the last string passed to
The subroutine returns 0 if the string s was compiled successfully; otherwise a string containing an error message is returned. If is
passed 0 or a null string, it returns without changing the currently compiled regular expression.
The subroutine returns 1 if the string s matches the last compiled regular expression, 0 if the string s failed to match the last compiled
regular expression, and -1 if the compiled regular expression was invalid (indicating an internal error).
The strings passed to both and may have trailing or embedded newline characters; they are terminated by nulls. The regular expressions
recognized are described in the manual entry for given the above difference.
Diagnostics
The subroutine returns -1 for an internal error.
The subroutine returns one of the following strings if an error occurs:
No previous regular expression
Regular expression too long
unmatched (
missing ]
too many () pairs
unmatched )
See Also
ed(1), ex(1), egrep(1), fgrep(1), grep(1)
regex(3)