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    <p>All the <code>Text.Regex.*</code> modules make heavy use of typeclasses, which are there for extensibility and "overloading"-like behavior, but make usage less obvious from just seeing types.</p> <p>Now, you've probably been started off from the basic <code>=~</code> matcher.</p> <pre><code>(=~) :: ( RegexMaker Regex CompOption ExecOption source , RegexContext Regex source1 target ) =&gt; source1 -&gt; source -&gt; target (=~~) :: ( RegexMaker Regex CompOption ExecOption source , RegexContext Regex source1 target, Monad m ) =&gt; source1 -&gt; source -&gt; m target </code></pre> <p>To use <code>=~</code>, there must exist an instance of <code>RegexMaker ...</code> for the LHS, and <code>RegexContext ...</code> for the RHS and result.</p> <pre><code>class RegexOptions regex compOpt execOpt | ... | regex -&gt; compOpt execOpt , compOpt -&gt; regex execOpt , execOpt -&gt; regex compOpt class RegexOptions regex compOpt execOpt =&gt; RegexMaker regex compOpt execOpt source | regex -&gt; compOpt execOpt , compOpt -&gt; regex execOpt , execOpt -&gt; regex compOpt where makeRegex :: source -&gt; regex makeRegexOpts :: compOpt -&gt; execOpt -&gt; source -&gt; regex </code></pre> <p>A valid instance of all these classes (for example, <code>regex=Regex</code>, <code>compOpt=CompOption</code>, <code>execOpt=ExecOption</code>, and <code>source=String</code>) means it's possible to compile a <code>regex</code> with <code>compOpt,execOpt</code> options from some form <code>source</code>. (Also, given some <code>regex</code> type, there is exactly one <code>compOpt,execOpt</code> set that goes along with it. Lots of different <code>source</code> types are okay, though.)</p> <pre><code>class Extract source class Extract source =&gt; RegexLike regex source class RegexLike regex source =&gt; RegexContext regex source target where match :: regex -&gt; source -&gt; target matchM :: Monad m =&gt; regex -&gt; source -&gt; m target </code></pre> <p>A valid instance of all these classes (for example, <code>regex=Regex</code>, <code>source=String</code>, <code>target=Bool</code>) means it's possible to match a <code>source</code> and a <code>regex</code> to yield a <code>target</code>. (Other valid <code>target</code>s given these specific <code>regex</code> and <code>source</code> are <code>Int</code>, <code>MatchResult String</code>, <code>MatchArray</code>, etc.)</p> <p>Put these together and it's pretty obvious that <code>=~</code> and <code>=~~</code> are simply convenience functions</p> <pre><code>source1 =~ source = match (makeRegex source) source1 source1 =~~ source = matchM (makeRegex source) source1 </code></pre> <p>and also that <code>=~</code> and <code>=~~</code> leave no room to pass various options to <code>makeRegexOpts</code>.</p> <p>You could make your own</p> <pre><code>(=~+) :: ( RegexMaker regex compOpt execOpt source , RegexContext regex source1 target ) =&gt; source1 -&gt; (source, compOpt, execOpt) -&gt; target source1 =~+ (source, compOpt, execOpt) = match (makeRegexOpts compOpt execOpt source) source1 (=~~+) :: ( RegexMaker regex compOpt execOpt source , RegexContext regex source1 target, Monad m ) =&gt; source1 -&gt; (source, compOpt, execOpt) -&gt; m target source1 =~~+ (source, compOpt, execOpt) = matchM (makeRegexOpts compOpt execOpt source) source1 </code></pre> <p>which could be used like</p> <pre><code>"string" =~+ ("regex", CompCaseless + compUTF8, execBlank) :: Bool </code></pre> <p>or overwrite <code>=~</code> and <code>=~~</code> with methods which can accept options</p> <pre><code>import Text.Regex.PCRE hiding ((=~), (=~~)) class RegexSourceLike regex source where makeRegexWith source :: source -&gt; regex instance RegexMaker regex compOpt execOpt source =&gt; RegexSourceLike regex source where makeRegexWith = makeRegex instance RegexMaker regex compOpt execOpt source =&gt; RegexSourceLike regex (source, compOpt, execOpt) where makeRegexWith (source, compOpt, execOpt) = makeRegexOpts compOpt execOpt source source1 =~ source = match (makeRegexWith source) source1 source1 =~~ source = matchM (makeRegexWith source) source1 </code></pre> <p>or you could just use <code>match</code>, <code>makeRegexOpts</code>, etc. directly where needed.</p>
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    1. COAh, it seems that I've been beat to the solution. That's what I get for writing all sorts of unnecessary stuff :-/
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    2. COAh, I feel a little guilty now, yours certainly offers a much more comprehensive overview! I like you suggestion for (=~+) by the way.
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    3. COit's a very complete and comprehensive answer indeed, I'd like to reward the effort, but I don't know if it's common practice to switch the "accepted answer" ? anyway, I'm new to Haskell, and this answer really helped me to understand some clever principles of the language (also, little typo at the begining you wrote =~ instead of =~~ )
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