Using a variable in a regular expression

Question

I am using some sub(), gsub() functions to replace content stored in a variable.

Say for example:

$ awk 'BEGIN {str="hello bee"; patt="llo"; gsub(patt,"XX",str); print str}'
heXX bee

This replaces in the string contained in str all occurrences of llo with XX. Nice.

Now say that I want to use a more complex regular expression that uses both a variable and a pattern. For example, "he" + variable patt. How can I do it?

If I try with gsub(/he/patt, ...) it doesn't work:

awk 'BEGIN {str="hello bee"; patt="llo"; gsub(/he/patt,"XX",str); print str}'

Returns

hello bee

Instead of

XX bee

Added the [gawk] tag since it is the one I am using. Any solution using another distribution is also more than welcome : ) — fedorqui
– fedorqui, Commented Oct 1, 2015 at 15:06

Ed Morton · Accepted Answer · 2015-10-01 15:08:33Z

4

That's when you need string concatenation and so have to use string delimiters instead of regexp delimiters for the regexp:

$ awk 'BEGIN {str="hello bee"; patt="llo"; gsub("he"patt,"XX",str); print str}'
XX bee

Just be aware that you'll then need to double up any escape chars since awk has to convert the string to a regexp first and so that will use up one of the escape chars.

answered Oct 1, 2015 at 15:08

Ed Morton

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2 Comments

fedorqui Over a year ago

Nice! So for example I need to say gsub("he"patt"\\s","XX",str), that is, use \\s instead of \s to match the space.

Ed Morton Over a year ago

Correct. If you use a single escape, \s, then gawk will give you a warning about the \s being treated as a literal s and then do so but some other awks won't warn you and IIRC some will interpret it as if you wrote \\s while others will treat it as s.

anubhava · Accepted Answer · 2015-10-01 15:07:52Z

3

You can do:

awk 'BEGIN {str="hello bee"; patt="llo"; gsub("he" patt,"XX",str); print str}'
XX bee

/he/patt doesn't concat the string together but "he" patt does.

answered Oct 1, 2015 at 15:07

anubhava

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2 Comments

fedorqui Over a year ago

Oh, nice! So there is no way to concat a string using /he/?

Ed Morton Over a year ago

@fedorui /he/ is not a string, its a regexp. There is no such thing as regexp concatenation, only string concatenation and conversion of strings to regexps when strings are used in a regexp context.

karakfa · Accepted Answer · 2015-10-01 15:07:52Z

0

this will work

awk 'BEGIN {str="hello bee"; patt="llo"; gsub(patt,"XX",str); print str}'
heXX bee

essentially you have to define the full pattern as a variable, not part of it.

answered Oct 1, 2015 at 15:07

karakfa

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1 Comment

fedorqui Over a year ago

Yep! This is a way, and thanks for it. Note, though, that I am wondering if there is a way to do such concatenation.

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