Bash - Regex for HTML contents - regex

Question

I'm learning about Bash scripting, and need some help understanding regex's.
I have a variable that is basically the html of a webpage (exported using wget):
currentURL = "https://www.example.com"
currentPage=$(wget -q -O - $currentURL)
I want to get the id's of all linked photos in this page. I just need help figuring out what the RegEx should be.
I started with this, but I need to modify the regex:
Test string (this is what currentURL contains, there can be zero to many instances of this):
<img src="./download/file.php?id=123456&t=1">
Current Regex:
.\/download\/file.php\?id=[0-9]{6}\&mode=view
Here's the regex I created, but it doesn't seem to work in bash.
The best solution would be to have the ID of each file. In this case, simply 123456. But if we can start with getting the /download/file.php?id=123456, that'd be a good start.

Answer 1

Don't parse XML/HTML with regex, use a proper XML/HTML parser.
theory :
According to the compiling theory, HTML can't be parsed using regex based on finite state machine. Due to hierarchical construction of HTML you need to use a pushdown automaton and manipulate LALR grammar using tool like YACC.
realLife©®™ everyday tool in a shell :
You can use one of the following :
xmllint often installed by default with libxml2, xpath1
xmlstarlet can edit, select, transform... Not installed by default, xpath1
xpath installed via perl's module XML::XPath, xpath1
xidel xpath3
saxon-lint my own project, wrapper over #Michael Kay's Saxon-HE Java library, xpath3
or you can use high level languages and proper libs, I think of :
python's lxml (from lxml import etree)
perl's XML::LibXML, XML::XPath, XML::Twig::XPath, HTML::TreeBuilder::XPath
Check: Using regular expressions with HTML tags
Example using xidel:
xidel -s "$currentURL" -e '//a/extract(#href,"id=(\d+)",1)'

Answer 2

Let's first clarify a couple of misunderstandings.
I'm learning about Bash scripting, and need some help understanding regex's.
You seem to be implying some sort of relation between Bash and regex.
As if Bash was some sort of regex engine.
It isn't. The [[ builtin is the only thing I recall in Bash that supports regular expressions, but I think you mean something else.
There are some common commands executed in Bash that support some implementation of regular expressions such as grep or sed and others. Maybe that's what you meant. It's good to be specific and accurate.
I want to get the id's of all linked photos in this page. I just need help figuring out what the RegEx should be.
This suggests an underlying assumption that if you want to extract content from an HTML, then regex is the way to go. That assumption is incorrect.
Although it's best to extract content from HTML using an XML parser (using one of the suggestions in Gilles' answer),
and trying to use regex for it is not a good reflect,
for simple cases like yours it might just be good enough:
grep -oP '\./download/file\.php\?id=\K\d+(?=&mode=view)' file.html
Take note that you escaped the wrong characters in the regex:
/ and & don't have a special meaning and don't need to be escaped
. and ? have special meaning and need to be escaped
Some extra tricks in the above regex are good to explain:
The -P flag of grep enables Perl style (powerful) regular expressions
\K is a Perl specific symbol, it means to not include in the match the content before the \K
The (?=...) is a zero-width positive lookahead assertion. For example, /\w+(?=\t)/ matches a word followed by a tab, without including the tab in the match.
The \K and the lookahead trickery is to work with grep -o, which outputs only the matched part. But without these trickeries the matched part would be for example ./download/file.php?id=123456&mode=view, which is more than what you want.