Let's try this. It's BUGGERED.

src/cmd/eqn/lex.c

@@ -0,0 +1,265 @@
+#include "e.h"
+#include "y.tab.h"
+#include <ctype.h>
+
+#define	SSIZE	1000
+char	token[SSIZE];
+int	sp;
+
+void	space(void);
+void	dodef(tbl *);
+void	define(int);
+void	ifdef(void);
+void	include(void);
+void	delim(void);
+
+yylex(void)
+{
+	register int c;
+	tbl *tp;
+
+  begin:
+	while ((c = input()) == ' ' || c == '\n' || c == '\t')
+		;
+	yylval = c;
+	switch (c) {
+	case EOF:
+		ERROR "unexpected end of input inside equation" WARNING;
+		return(EOF);
+	case '~':
+		return(SPACE);
+	case '^':
+		return(THIN);
+	/* case '\t':
+		return(TAB);
+	*/
+	case '{':
+		return('{');
+	case '}':
+		return('}');
+	case '"':
+		for (sp = 0; (c=input())!='"' && c != '\n'; ) {
+			if (c == '\\')
+				if ((c = input()) != '"')
+					token[sp++] = '\\';
+			token[sp++] = c;
+			if (sp >= SSIZE)
+				ERROR "quoted string %.20s... too long", token FATAL;
+		}
+		token[sp] = '\0';
+		yylval = (int) &token[0];
+		if (c == '\n')
+			ERROR "missing \" in %.20s", token WARNING;
+		return(QTEXT);
+	}
+	if (!display && c == righteq)
+		return(EOF);
+
+	unput(c);
+	getstr(token, SSIZE);
+	dprintf(".\tlex token = |%s|\n", token);
+	if ((tp = lookup(deftbl, token)) != NULL) {	/* defined term */
+		c = input();
+		unput(c);
+		if (c == '(')	/* macro with args */
+			dodef(tp);
+		else {		/* no args */
+			unput(' ');
+			pbstr(tp->cval);
+			dprintf(".\tfound %s|=%s|\n", token, tp->cval);
+		}
+		goto begin;
+	}
+
+	if ((tp = lookup(keytbl, token)) == NULL)	/* not a keyword */
+		return CONTIG;
+
+	switch (tp->ival) {		/* some kind of keyword */
+	case DEFINE: case TDEFINE: case NDEFINE:
+		define(tp->ival);
+		break;
+	case IFDEF:
+		ifdef();
+		break;
+	case DELIM:
+		delim();
+		break;
+	case GSIZE:
+		globsize();
+		break;
+	case GFONT:
+		globfont();
+		break;
+	case INCLUDE:
+		include();
+		break;
+	case SPACE:
+		space();
+		break;
+	case DOTEQ:
+			/* .EQ inside equation -- should warn if at bottom level */
+		break;
+	case DOTEN:
+		if (curfile == infile)
+			return EOF;
+		/* else ignore nested .EN */
+		break;
+	default:
+		return tp->ival;
+	}
+	goto begin;
+}
+
+void getstr(char *s, int n)
+{
+	register int c;
+	register char *p;
+
+	p = s;
+	while ((c = input()) == ' ' || c == '\n')
+		;
+	if (c == EOF) {
+		*s = 0;
+		return;
+	}
+	while (c != ' ' && c != '\t' && c != '\n' && c != '{' && c != '}'
+	    && c != '"' && c != '~' && c != '^') {
+		if (!display && c == righteq)
+			break;
+		if (c == '(' && p > s) {	/* might be defined(...) */
+			*p = '\0';
+			if (lookup(deftbl, s) != NULL)
+				break;
+		}
+		if (c == '\\')
+			if ((c = input()) != '"')
+				*p++ = '\\';
+		*p++ = c;
+		if (--n <= 0)
+			ERROR "token %.20s... too long", s FATAL;
+		c = input();
+	}
+	unput(c);
+	*p = '\0';
+	yylval = (int) s;
+}
+
+cstr(char *s, int quote, int maxs)
+{
+	int del, c, i;
+
+	s[0] = 0;
+	while ((del=input()) == ' ' || del == '\t')
+		;
+	if (quote)
+		for (i=0; (c=input()) != del && c != EOF;) {
+			s[i++] = c;
+			if (i >= maxs)
+				return(1);	/* disaster */
+		}
+	else {
+		if (del == '\n')
+			return(1);
+		s[0] = del;
+		for (i=1; (c=input())!=' ' && c!= '\t' && c!='\n' && c!=EOF;) {
+			s[i++] = c;
+			if (i >= maxs)
+				return(1);	/* disaster */
+		}
+	}
+	s[i] = '\0';
+	if (c == EOF)
+		ERROR "Unexpected end of input at %.20s", s FATAL;
+	return(0);
+}
+
+void define(int type)
+{
+	char *p1, *p2;
+	extern int ftune(char *, char *);
+
+	getstr(token, SSIZE);	/* get name */
+	if (type != DEFINE) {
+		cstr(token, 1, SSIZE);	/* skip the definition too */
+		return;
+	}
+	p1 = strsave(token);
+	if (cstr(token, 1, SSIZE))
+		ERROR "Unterminated definition at %.20s", token FATAL;
+	if (lookup(ftunetbl, p1) != NULL) {	/* double tuning param */
+		dprintf(".\ttune %s %s\n", p1, token);
+		ftune(p1, token);
+	} else {
+		p2 = strsave(token);
+		install(deftbl, p1, p2, 0);
+		dprintf(".\tname %s defined as %s\n", p1, p2);
+	}
+}
+
+void ifdef(void)		/* do body if name is defined */
+{
+	char name[100], *p;
+
+	getstr(name, sizeof(name));	/* get name */
+	cstr(token, 1, SSIZE);		/* and body */
+	if (lookup(deftbl, name) != NULL) {	/* found it */
+		p = strsave(token);
+		pushsrc(Free, p);
+		pushsrc(String, p);
+	}
+}
+
+char	*spaceval	= NULL;
+
+void space(void)	/* collect line of form "space amt" to replace \x in output */
+{
+	getstr(token, SSIZE);
+	spaceval = strsave(token);
+	dprintf(".\tsetting spaceval to %s\n", token);
+}
+
+char *strsave(char *s)
+{
+	register char *q;
+
+	q = malloc(strlen(s)+1);
+	if (q == NULL)
+		ERROR "out of space in strsave on %s", s FATAL;
+	strcpy(q, s);
+	return(q);
+}
+
+void include(void)
+{
+	char name[100];
+	FILE *fin;
+	int c;
+	extern int errno;
+
+	while ((c = input()) == ' ')
+		;
+	unput(c);
+	cstr(name, c == '"', sizeof(name));	/* gets it quoted or not */
+	if ((fin = fopen(name, "r")) == NULL)
+		ERROR "can't open file %s", name FATAL;
+	errno = 0;
+	curfile++;
+	curfile->fin = fin;
+	curfile->fname = strsave(name);
+	curfile->lineno = 0;
+	printf(".lf 1 %s\n", curfile->fname);
+	pushsrc(File, curfile->fname);
+}
+
+void delim(void)
+{
+	yyval = eqnreg = 0;
+	if (cstr(token, 0, SSIZE))
+		ERROR "Bizarre delimiters" FATAL;
+	lefteq = token[0];
+	righteq = token[1];
+        if (!isprint(lefteq) || !isprint(righteq))
+		ERROR "Bizarre delimiters" FATAL;
+	if (lefteq == 'o' && righteq == 'f')
+		lefteq = righteq = '\0';
+}