regexp, help with assertions - regex

Question

I have the following string:
<a name="subhd_182"></a>
<a name="st_394"></a>
<a name="st_395"></a>
<a name="qn_494"></a>
<a name="st_495"></a>
<a name="qn_594"></a>
<a name="st_595"></a>
<a name="subhd_282"></a>
<a name="qn_694"></a>
<a name="st_695"></a>
<a name="qn_794"></a>
<a name="st_795"></a>
<a name="qn_894"></a>
<a name="st_895"></a>`
And I want to replace every <a name="st_\d*"></a> with <a name="qn_\d*"></a> if it follows immediately <a name="subhd_\d*"></a>
I use this regex %(.*<a name="subhd_.*)(?=<a name="st(?!<a name="qn))(<a name=")st(.*)%sU and replace with $1$2qn$3. But it also replaces second case too

Answer 1

I'm assuming you only want to match name after the first subhd row above, but not the second, since the first one is an "st_" and the second one is a "qn_".
Try:
(<a name="subhd_\d+">\s*<\/a>\s*<a name=")st(_\d+">)
where you would replace as $1qn$2 Note that here I have assumed that you were quite literal when you said "it follows immediately .
I don't really understand why you're throwing the lookahead in, unless the actual rule you're trying to implement is more complicated than you've stated.

Answer 2

Try: %(<a name="subhd_\d+"></a>\n<a name=")st(.*)%sU and replace with $1qn$2. On a sidenote I don't really know what the U modifier does for you here. Also, you might want to change your \n newline matcher according to your operating system.
I have found RegExr a really useful tool for regular expressions.