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String Operators

RegEx

This uses golangs native regex functions under the hood - See their docs for the supported syntax.
Case insensitive tip: prefix the regex with (?i) - e.g. test("(?i)cats)".

match(regEx)

This operator returns the substring match details of the given regEx.

capture(regEx)

Capture returns named RegEx capture groups in a map. Can be more convenient than match depending on what you are doing.

test(regEx)

Returns true if the string matches the RegEx, false otherwise.

sub(regEx, replacement)

Substitutes matched substrings. The first parameter is the regEx to match substrings within the original string. The second is a what to replace those matches with. This can refer to capture groups from the first RegEx.

String blocks, bash and newlines

Bash is notorious for chomping on precious trailing newline characters, making it tricky to set strings with newlines properly. In particular, the $( exp ) will trim trailing newlines.
For instance to get this yaml:
a: |
cat
Using $( exp ) wont work, as it will trim the trailing new line.
m=$(echo "cat\n") yq -n '.a = strenv(m)'
a: cat
However, using printf works:
printf -v m "cat\n" ; m="$m" yq -n '.a = strenv(m)'
a: |
cat
As well as having multiline expressions:
m="cat
" yq -n '.a = strenv(m)'
a: |
cat
Similarly, if you're trying to set the content from a file, and want a trailing new line:
IFS= read -rd '' output < <(cat my_file)
output=$output ./yq '.data.values = strenv(output)' first.yml

To up (upper) case

Works with unicode characters
Given a sample.yml file of:
água
then
yq 'upcase' sample.yml
will output
ÁGUA

To down (lower) case

Works with unicode characters
Given a sample.yml file of:
ÁgUA
then
yq 'downcase' sample.yml
will output
água

Join strings

Given a sample.yml file of:
- cat
- meow
- 1
- null
- true
then
yq 'join("; ")' sample.yml
will output
cat; meow; 1; ; true

Trim strings

Given a sample.yml file of:
- ' cat'
- 'dog '
- ' cow cow '
- horse
then
yq '.[] | trim' sample.yml
will output
cat
dog
cow cow
horse

Match string

Given a sample.yml file of:
foo bar foo
then
yq 'match("foo")' sample.yml
will output
string: foo
offset: 0
length: 3
captures: []

Match string, case insensitive

Given a sample.yml file of:
foo bar FOO
then
yq '[match("(?i)foo"; "g")]' sample.yml
will output
- string: foo
offset: 0
length: 3
captures: []
- string: FOO
offset: 8
length: 3
captures: []

Match with global capture group

Given a sample.yml file of:
abc abc
then
yq '[match("(ab)(c)"; "g")]' sample.yml
will output
- string: abc
offset: 0
length: 3
captures:
- string: ab
offset: 0
length: 2
- string: c
offset: 2
length: 1
- string: abc
offset: 4
length: 3
captures:
- string: ab
offset: 4
length: 2
- string: c
offset: 6
length: 1

Match with named capture groups

Given a sample.yml file of:
foo bar foo foo foo
then
yq '[match("foo (?P<bar123>bar)? foo"; "g")]' sample.yml
will output
- string: foo bar foo
offset: 0
length: 11
captures:
- string: bar
offset: 4
length: 3
name: bar123
- string: foo foo
offset: 12
length: 8
captures:
- string: null
offset: -1
length: 0
name: bar123

Capture named groups into a map

Given a sample.yml file of:
xyzzy-14
then
yq 'capture("(?P<a>[a-z]+)-(?P<n>[0-9]+)")' sample.yml
will output
a: xyzzy
n: "14"

Match without global flag

Given a sample.yml file of:
cat cat
then
yq 'match("cat")' sample.yml
will output
string: cat
offset: 0
length: 3
captures: []

Match with global flag

Given a sample.yml file of:
cat cat
then
yq '[match("cat"; "g")]' sample.yml
will output
- string: cat
offset: 0
length: 3
captures: []
- string: cat
offset: 4
length: 3
captures: []

Test using regex

Like jq's equivalent, this works like match but only returns true/false instead of full match details
Given a sample.yml file of:
- cat
- dog
then
yq '.[] | test("at")' sample.yml
will output
true
false

Substitute / Replace string

This uses golang regex, described here Note the use of |= to run in context of the current string value.
Given a sample.yml file of:
a: dogs are great
then
yq '.a |= sub("dogs", "cats")' sample.yml
will output
a: cats are great

Substitute / Replace string with regex

This uses golang regex, described here Note the use of |= to run in context of the current string value.
Given a sample.yml file of:
a: cat
b: heat
then
yq '.[] |= sub("(a)", "${1}r")' sample.yml
will output
a: cart
b: heart

Custom types: that are really strings

When custom tags are encountered, yq will try to decode the underlying type.
Given a sample.yml file of:
a: !horse cat
b: !goat heat
then
yq '.[] |= sub("(a)", "${1}r")' sample.yml
will output
a: !horse cart
b: !goat heart

Split strings

Given a sample.yml file of:
cat; meow; 1; ; true
then
yq 'split("; ")' sample.yml
will output
- cat
- meow
- "1"
- ""
- "true"

Split strings one match

Given a sample.yml file of:
word
then
yq 'split("; ")' sample.yml
will output
- word