What does HTML Parsing mean?
I have heard of HTML Parser libraries like Simple HTML DOM and HTML Parser. I have also heard of questions containing HTML Parsing. What does it mean to parse HTML?
Answer
Unlike what Spudley said, parsing is basically to resolve (a sentence) into its component parts and describe their syntactic roles.
According to wikipedia, Parsing or syntactic analysis is the process of analysing a string of symbols, either in natural language or in computer languages, according to the rules of a formal grammar. The term parsing comes from Latin pars (orationis), meaning part (of speech).
In your case, HTML parsing is basically: taking in HTML code and extracting relevant information like the title of the page, paragraphs in the page, headings in the page, links, bold text etc.
Parsers:
A computer program that parses content is called a parser. There are in general 2 kinds of parsers:
Top-down parsing- Top-down parsing can be viewed as an attempt to find left-most derivations of an input-stream by searching for parse trees using a top-down expansion of the given formal grammar rules. Tokens are consumed from left to right. Inclusive choice is used to accommodate ambiguity by expanding all alternative right-hand-sides of grammar rules.
Bottom-up parsing - A parser can start with the input and attempt to rewrite it to the start symbol. Intuitively, the parser attempts to locate the most basic elements, then the elements containing these, and so on. LR parsers are examples of bottom-up parsers. Another term used for this type of parser is Shift-Reduce parsing.
A few example parsers:
Top-down parsers:
Bottom-up parsers:
- Precedence parser
- BC (bounded context) parsing
- LR parser (Left-to-right, Rightmost derivation)
- Simple LR (SLR) parser
- LALR parser
- Canonical LR (LR(1)) parser
- GLR parser
- CYK parser
- Recursive ascent parser
Example parser:
Here's an example HTML parser in python:
from HTMLParser import HTMLParser
# create a subclass and override the handler methods
class MyHTMLParser(HTMLParser):
def handle_starttag(self, tag, attrs):
print "Encountered a start tag:", tag
def handle_endtag(self, tag):
print "Encountered an end tag :", tag
def handle_data(self, data):
print "Encountered some data :", data
# instantiate the parser and fed it some HTML
parser = MyHTMLParser()
parser.feed('<html><head><title>Test</title></head>'
'<body><h1>Parse me!</h1></body></html>')
Here's the output:
Encountered a start tag: html Encountered a start tag: head Encountered a start tag: title Encountered some data : Test Encountered an end tag : title Encountered an end tag : head Encountered a start tag: body Encountered a start tag: h1 Encountered some data : Parse me! Encountered an end tag : h1 Encountered an end tag : body Encountered an end tag : html