AWK reference

GNU Awk Manual

String functions

asort(source [, dest [, how ] ])		# Sort values and replace indices
gensub(regexp, replacement, how [, target])	# gsub with multiple matches in the replacement
gsub(regexp, replacement [, target])
index(in, find)
match(string, regexp [, array])			# Returns index of the match, or zero if not found
patsplit(string, array [, fieldpat [, seps ] ])	# Split by content like FPAT
split(string, array [, fieldsep [, seps ] ])
sprintf(format, expression1, ...)
sub(regexp, replacement [, target])
substr(string, start [, length ])

asort, gensub, patsplit and strtonum are not on POSIX, but busybox awk supports gensub.

Regular Expressions

Bracket Expressions:

[:alnum:]	# Alphanumeric characters
[:alpha:]	# Alphabetic characters
[:blank:]	# Space and TAB characters
[:cntrl:]	# Control characters
[:digit:]	# Numeric characters
[:graph:]	# Characters that are both printable and visible
[:lower:]	# Lowercase alphabetic characters
[:print:]	# Printable characters
[:punct:]	# Punctuation characters
[:space:]	# Space characters (such as space, TAB, and formfeed, to name a few)
[:upper:]	# Uppercase alphabetic characters
[:xdigit:]	# Characters that are hexadecimal digits

Time functions

mktime(datespec)		# Turn datespec into a timestamp
strftime(format, timestamp)	# Format the timestamp based on the format string
systime()			# Return the Unix time

Not on POSIX but both mawk and busybox support this functions.

Controlling Array Traversal

Sort by index/value string/numeric ascending/descending:

PROCINFO["sorted_in"] = "@ind_str_asc" # @ind_num_asc val_str_asc @val_num_asc @ind_str_desc
for (i in array)

Custom function:

comp_func(i1, v1, i2, v2) {
	return v1 - v2 # Sort by value
	return i1 - i2 # Sort by index
PROCINFO["sorted_in"] = "comp_func"
for (i in array)