Creating an alias for xpath expression in xidel with regex and bash

If you have already used Xidel, you will often need to locate nodes that have a certain class. To do this more easy, I want to create has-class("class") function that serves as an alias for the expression:
contains(concat(" ", normalize-space(@class), " "), " class ").

Example:

$ e-xidel.sh example.com '//article/p//img[has-class("wp-image")]'

e-xidel.sh contains this code:

#!/bin/bash

echo -e "$(tput setaf 2) Checking... $(tput sgr0)"

path=$1
expression=$2

# expression = '//article/p//img[has-class("wp-image")]'
# Regex to replace every * has-class("class") * by * contains(concat(" ", normalize-space(@class), " "), " class ") *
# ...
# ...
# expression = '//article/p//img[contains(concat(" ", normalize-space(@class), " "), " wp-image ")]'

xoutput=$(xidel $path --printed-node-format=html --output-declaration= -e "$expression")

echo -e "$(tput setaf 1) $xoutput $(tput sgr0)"

Solution

You can use sed (GNU version, cannot guarantee it will work with others implementations) to achieve your need:

sed 's/has-class("\([^)]\+\)")/contains(concat(" ", normalize-space(@class), " "), " \1 ")/g'

Explanation:

s/pattern/substitution/g: replace the portion matching the pattern by the substitution string; g flag for replace all the portions of line (global substitution)
has-class("\([^)]\+\)"): a portion starting with has-class(" containing any character except the closing parenthesis ([^)]) and ending by "). Escaped parentheses surrounding the inner part capture the subportion and associate it with the alias \1, since it's the first created capture group.
contains(concat(" ", normalize-space(@class), " "), " \1 "): replace the mached portion by this text; \1 will be expanded by the content of the associated captured group.

Your script would be:

#!/bin/bash

function expand-has-class() {
    echo "$1" |
    sed 's/has-class("\([^)]\+\)")/contains(concat(" ", normalize-space(@class), " "), " \1 ")/g'
}

echo -e "$(tput setaf 2) Checking... $(tput sgr0)"

path=$1
expression="$(expand-has-class "$2")"

# expression = '//article/p//img[has-class("wp-image")]'
# Regex to replace every * has-class("class") * by * contains(concat(" ", normalize-space(@class), " "), " class ") *
# ...
# ...
# expression = '//article/p//img[contains(concat(" ", normalize-space(@class), " "), " wp-image ")]'

xoutput=$(xidel $path --printed-node-format=html --output-declaration= -e "$expression")

echo -e "$(tput setaf 1) $xoutput $(tput sgr0)"