There are a couple of general rules I use to keep track of things.

• The engine prefers the leftmost, longest match. If it can match something early in the string or late in the string, it prefers the former.
• Using an anchor, obviously, changes this.
• The default quantifiers (* and +) are greedy. They prefer longer matches, and will happily gobble up everything in the string.
• All of these rules apply to each element of the regex. If the first element of the regex is greedy, it'll land at the end of the string. To match the next element, the engine will backtrack one character at a time to see if it can match.
• Internal optimizations in the engine can avoid some of the caveats these rules imply, but they tend not to change the rules.

I second the recommendation of Mastering Regular Expressions, though I hear Jeff's working on a new version at the moment.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.