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General programming in Awk

Reusing regular expression groups with awk by xmb
Posted: 12 Feb 05 (Edited 23 Feb 05)

Regular expression groups is stuff matched in brackets, eg /some([^:]+):here:(.*)/

awk, in general, cannot use or reuse those. They are normally processed as a match.

The only way to accomplish this (besides alternative techniques), is using gawk's gensub() and match() function

match() is extended by an optional third argument, a destination array in which the groups will get stored
gensub() is kindof an advanced g/sub() returning the new string. The group is specified by \\<number> in its second argument, \\0 represents the whole match, as does &. Its third argument indicates which match to actually replace, or "g" for all. Note on where .* is wanted/needed.

Note, awk does not support look-(ahead|behind), or any (<modifier><regex>) classes as PCRE or other advanced (standarized partly) libraries do, not even {<number>} counted matches, excepts gawk with --re-interval

# btw, my prompt looks like this: xmb (gp:4:3)~/awk $
# PS1='\u (\[\e[1;31m\]\h\[\e[m\]:\[\e[33m\]\l\[\e[m\]:\[\e[32m\]\j\[\e[m\])\[\e[1m\]\w\[\e[m\] \$ '
Examples:

CODE

$ echo 'From: "Sumone @home" <home@me.com>
From: malformed <doh>' |
    gawk '{ print gensub(/.*:[ "]+([^"]+| *<).*<([^>]+).*/, "\\1 -+- \\2", 1) }'
Sumone @home -+- home@me.com
malformed  -+- doh

CODE

$ echo XabF XcdF XdeF | gawk '{ print gensub(/X([^X]+)F/, "\\1", 2) }'
XabF cd XdeF

$ echo XabF XcdF XdeF | gawk '{ print gensub(/X([^X]+)F/, "\\1", "g") }'
ab cd de

CODE

$ echo "<html><head><blah foo=bar>yeah<..>" | gawk '{
    match($0, /head><([^ ]+) ([^=]+)=([^>]+)>([^<]+)/, Arr)
    printf "%s (%s->%s) == %s\n", Arr[1], Arr[2], Arr[3], Arr[4]
    }'
blah (foo->bar) == yeah

; [ng tag]

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