shell script how to extract string using regular expressions

Question

I am new to shell scripts  I want to send a http request using curl and then extract some string using regular expressions  For example  how can I extract a domain name from a http response   The example is for learning purposes only      bin bash name   curl google com   grep  www    com   echo  domain name is  echo  name

User · Answer

One way would be with sed   For example   echo  name   sed -e  s http   www        Normally the sed regular expressions are delimited by      but you can use     since you re searching for       Here s another bash trick   DigitalTrauma s answer reminded me that I ought to suggest it   It s similar   echo   name http   www      DigitalTrauma also gets credit for reminding me that the  http     needs to be handled

User · Answer

Using bash regular expressions   re  http             if     name     re     then echo   BASH REMATCH 1    fi   Edit - OP asked for explanation of syntax   Regular expression syntax is a large topic which I can t explain in full here  but I will attempt to explain enough to understand the example   re  http               This is the regular expression stored in a bash variable  re - i e  what you want your input string to match  and hopefully extract a substring   Breaking it down    http    is just a string - the input string must contain this substring for the regular expression to match    Normally square brackets are used say  match any character within the brackets    So c ao t would match both  cat  and  cot    The   character within the    modifies this to say  match any character except those within the square brackets   So in this case      will match any character apart from      The square bracket expression will only match one character   Adding a   to the end of it says  match 1 or more of the preceding sub-expression    So       matches 1 or more of the set of all characters  excluding      Putting    parentheses around a subexpression says that you want to save whatever matched that subexpression for later processing   If the language you are using supports this  it will provide some mechanism to retrieve these submatches   For bash  it is the BASH REMATCH array  Finally we do an exact match on     to make sure we match all the way to end of the fully qualified domain name and the following       Next  we have to test the input string against the regular expression to see if it matches   We can use a bash conditional to do that   if     name     re     then     echo   BASH REMATCH 1   fi   In bash  the       specify an extended conditional test  and may contain the    bash regular expression operator   In this case we test whether the input string  name matches the regular expression  re   If it does match  then due to the construction of the regular expression  we are guaranteed that we will have a submatch  from the parentheses      and we can access it using the BASH REMATCH array    Element 0 of this array   BASH REMATCH 0   will be the entire string matched by the regular expression  i e   http   www google com    Subsequent elements of this array will be subsequent results of submatches   Note you can have multiple submatch    within a regular expression - The BASH REMATCH elements will correspond to these in order   So in this case   BASH REMATCH 1   will contain  www google com   which I think is the string you want    Note that the contents of the BASH REMATCH array only apply to the last time the regular expression    operator was used   So if you go on to do more regular expression matches  you must save the contents you need from this array each time   This may seem like a lengthy description  but I have really glossed over several of the intricacies of regular expressions   They can be quite powerful  and I believe with decent performance  but the regular expression syntax is complex   Also regular expression implementations vary  so different languages will support different features and may have subtle differences in syntax   In particular escaping of characters within a regular expression can be a thorny issue  especially when those characters would have an otherwise different meaning in the given language     Note that instead of setting the  re variable on a separate line and referring to this variable in the condition  you can put the regular expression directly into the condition   However in bash 3 2  the rules were changed regarding whether quotes around such literal regular expressions are required or not   Putting the regular expression in a separate variable is a straightforward way around this  so that the condition works as expected in all bash versions that support the    match operator

[regex] shell script. how to extract string using regular expressions

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